Compositions comprising free-standing two-dimensional nanocrystals

ABSTRACT

The present invention is directed to compositions comprising free standing and stacked assemblies of two dimensional crystalline solids, and methods of making the same.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part of PCT/US2012/043273, filedJun. 20, 2012, which claims priority to U.S. Provisional ApplicationSer. Nos. 61/499,318; 61/521,428; and 61/587,172, filed Jun. 21, 2011,Aug. 9, 2011, and Jan. 17, 2012, respectively. This application alsoclaims the benefit of priority to U.S. Provisional Application Ser. No.61/733,015, filed Dec. 4, 2012. The subject matter of each of theseapplications is incorporated by reference herein in its entirety for allpurposes.

GOVERNMENT INTERESTS

This invention was made with government support under a grant from theU.S. Department of Energy under Contract No. DE-AC02-05CH11231,Subcontract 6951370. The government has certain rights in the invention.

TECHNICAL FIELD

The present invention is directed to compositions comprising freestanding two dimensional crystalline solids, and methods of making thesame.

BACKGROUND

Typically, two-dimensional, 2-D, free-standing crystals exhibitproperties that differ from those of their three-dimensional, 3-D,counterparts. Currently, however, there are relatively few materialswhich can be described as 2-D, atomically-scaled layered solids. Clearlythe most studied freestanding 2-D material is graphene, but othermaterials include hexagonal BN, certain transition metal oxides,hydroxides, and silicates, including clays, S₂N, MoS₂ and WS₂ are alsoknown. Currently, the number of non-oxide materials that have beenexfoliated is limited to two fairly small groups, viz. hexagonal, vander Waals bonded structures (e.g. graphene and BN) and layered metalchalcogenides (e.g. MoS₂, WS₂, etc.).

Although graphene has attracted more attention than all other 2-Dmaterials together, its simple chemistry and the weak van der Waalsbonding between layers in multi-layer structures limit its use. Giventhe properties of graphene for applications ranging from compositereinforcement to electronics, there is interest in other new materialswhich may also be described as 2-D, atomically-scaled layered solids.

SUMMARY

This invention is directed to compositions comprising free standing andstacked assemblies of two dimensional crystalline solids, and methods ofmaking the same.

Various embodiments of this invention provide compositions comprising atleast one layer having first and second surfaces, each layer comprising:

a substantially two-dimensional array of crystal cells,

each crystal cell having an empirical formula of M_(n+1)X_(n), such thateach X is positioned within an octahedral array of M,

wherein M is at least one Group IIIB, IVB, VB, or VIB metal;

wherein each X is C and/or N (i.e., stoichiometrically X═C_(x)N_(y),including where x+y=1); and

n=1, 2, or 3.

Various embodiments provide for compositions composed of individual or aplurality of such layers.

Other embodiments provide that at least one of the surfaces is coatedwith a coating comprising alkoxide, carboxylate, halide, hydroxide,oxide, sub-oxide, nitride, sub-nitride, sulfide, thiol, or a combinationthereof.

Still further embodiments provide polymer composites comprising anorganic polymer and at least one composition described in the precedingparagraphs.

Certain embodiments provide for at least one stacked assembly of atleast two layers having first and second surfaces, each layercomprising:

a substantially two-dimensional array of crystal cells,

each crystal cell having the empirical formula of M_(n+1)X_(n), suchthat each X is positioned within an octahedral array of M;

wherein M is a Group IIIB, IVB, VB, or VIB metal;

each X is C and/or N (i.e., stoichiometrically X═C_(x)N_(y), includingwhere x+y=1); and

n=1, 2, or 3;

wherein the layers are characterized as having an average surface areaand interlayer distance.

In other embodiments, at least one of the surfaces of the layers withina stacked assembly has bound thereto alkoxide, carboxylate, halide,hydroxide, oxide, sub-oxide, nitride, sub-nitride, sulfide, thiol, or acombination thereof.

In some embodiments, the stacked assemblies described in the precedingparagraphs are capable of, or have atoms or ions, that are intercalatedbetween at least some of the layers. In other embodiments, these atomsor ions are lithium. In still other embodiments, these structures arepart of an energy storing device or a battery.

This invention also describes methods of preparing compositionscomprising: removing substantially all of the A atoms from a MAX-phasecomposition having an empirical formula of M_(n+1)AX_(n).

wherein M is at least one Group IIIB, IVB, VB, or VIB metal,

wherein A is an A-group element;

each X is C and/or N (i.e., stoichiometrically X═C_(x)N_(y), includingwhere x+y=1); and

n=1, 2, or 3,

thereby providing a composition comprising at least one layer having afirst and second surface, each layer comprising a substantiallytwo-dimensional array of crystal cells.

BRIEF DESCRIPTION OF THE DRAWINGS

The following figures are presented as illustrative examples, and shouldnot be considered to limit the scope of the invention in any way. Exceptwhere otherwise noted, the scales of the figures may be exaggerated forillustrative purposes.

FIGS. 1A-C illustrate indicative crystal structures of the MAX phases inwhich the transitional metal carbide or nitride (M_(n+1)X_(n)) layersare interleaved with layers of pure A-group element. FIG. 1A illustratesthe configuration of the M₂X framework within the 211 class of MAX phasecompounds, wherein every third layer is A-group. FIG. 1B illustrates theconfiguration of the M₃X₂ framework within the 312 class of MAX phasecompounds, wherein every fourth layer is A-group. FIG. 1C illustratesthe configuration of the M₄X₃ framework within the 413 class of MAXphase compounds, wherein every fifth layer is A-group.

FIG. 2 provides a 3-dimensional (FIG. 2A) and 2-dimensional (FIG. 2B)representation of the crystal structure of the 312 class of compounds

FIG. 3A-C illustrate a schematic representation of the exfoliationprocess for Ti₃AlC₂. FIG. 3A provides the Ti₃AlC₂ structure. FIG. 3Bdiagrammatically illustrates a structure where the Al atoms have beenreplaced by OH after reaction with HF. FIG. 3C illustrates a structureresulting from the subsequent breakage of the hydrogen bonds andseparation of nano-sheets after sonication in methanol.

FIG. 4 provides analytical date of the Ti₃AlC₂ before and afterexfoliation. FIG. 4( a) provides an XRD pattern for Ti₃AlC₂ before anytreatment, simulated XRD patterns of Ti₃C₂F₂ and Ti₃C₂(OH)₂, measuredXRD patterns of Ti₃AlC₂ after HF treatment, and exfoliated nanosheetsproduced by sonication. FIG. 4( b) shows Raman spectra of Ti₃AlC₂ beforeand after HF treatment. FIG. 4( c) provides XPS spectra of Ti₃AlC₂before and after HF treatment. FIG. 4( d) provides an SEM image of asample after HF treatment. FIG. 4( e) shows a cold processed 25 mm diskof etched and exfoliated material after HF treatment.

FIG. 5 shows micrographs of exfoliated MXene nanosheets. FIG. 5( a)shows TEM micrographs of exfoliated 2-D nanosheets of Ti—C—O—F. FIG. 5(b) shows TEM micrographs of exfoliated 2-D nanosheets; inset selectedarea diffraction, SAD, shows hexagonal basal plane. FIG. 5( c) shows TEMmicrographs of single and double layer MXene sheets. FIG. 5( d) shows anHRTEM image showing the separation of individual sheets aftersonication. FIG. 5( e) shows an HRTEM image of bilayerTi₃C₂(OH)_(x)F_(y) (alternatively, Ti₃C₂T_(s)). FIG. 5( f) shows anatomistic model of the layer structure shown in FIG. 5( e). FIG. 5( g)shows a calculated band structure of single layer MXene with —OH and —Fsurface termination and no termination (Ti₃C₂), showing a change frommetal to semiconductor as a result of change in the surface chemistry.

FIG. 6 provides TEM images and simulated structures of multi-layerMXene. FIG. 6( a) provides TEM micrographs for stacked layers ofTi—C—O—F. Those are similar to multilayer graphene or exfoliatedgraphite that finds use in electrochemical storage. FIG. 6( b) providesTEM micrographs for the same stacked layers FIG. 6( a) but at a highermagnification. FIG. 6( c) provides a model of the Li-intercalatedstructure of Ti₃C₂ (Ti₃C₂Li₂). FIG. 6( d) provides TEM micrographs for aconical scroll of about 20 nm in outer diameter. FIG. 6( e) provides across sectional TEM image of a scroll with inner radius less than 20 nm.FIG. 6( f) provides a schematic representation of an MXene scroll(OH-terminated).

FIG. 7 provides X-ray diffraction patterns of (V_(1/2)Cr_(1/2))₃AlC₂before and after exfoliation. From the bottom up, FIG. 7 provides asimulated XRD pattern for (V_(1/2)Cr_(1/2))₃AlC₂ as determined byCrystalMaker software, and measured XRD patterns for powdered(V_(1/2)Cr_(1/2))₃AlC₂ before any treatment, (V_(1/2)Cr_(1/2))₃AlC₂after HF treatment, (V_(1/2)Cr_(1/2))₃AlC₂ after HF treatment and coldpressing (CP), and (V_(1/2)Cr_(1/2))₃AlC₂ before any treatment or coldpressing. The peak at 2θ=7.3° appears only after HF treatment and coldpressing. By analogy to the results shown for Ti₃AlC₂, this peak isattributed to (V_(1/2)Cr_(1/2))₃C₂. See Example 5.

FIG. 8 shows SEM micrographs and XRD spectra of chemically exfoliatedTi₂AlC.

FIG. 9 shows a secondary electron SEM micrograph for TiNbAlC after HFtreatment and XRD patterns before and after HF treatment at roomtemperature for TiNbAlC (50% HF 28 hrs) (inset is the XRD for(V_(0.5)Cr_(0.5))AlC₂; 50% HF 65 hrs, and cold pressed zoomed-in on the(0002) peak).

FIG. 10 shows SEM micrographs and XRD spectra of chemically exfoliatedTi₄AlC₃ (50% HF 72 hours at RT).

FIG. 11 shows SEM micrographs and XRD spectra of chemically exfoliatedTi₃AlCN (30% HF 18 hours at RT).

FIG. 12 shows additional SEM micrographs of chemically exfoliatedTi₃AlCN (30% HF 18 hours at RT).

FIG. 13 shows additional SEM micrographs of chemically exfoliatedTi₃AlCN (30% HF 18 hours at RT).

FIG. 14 shows SEM micrographs and XRD spectra of chemically exfoliated(V_(1/2)Cr_(1/2))₃AlC₂ (50% HF 69 hours at RT).

FIG. 15 shows additional SEM micrographs of chemically exfoliated(V_(1/2)Cr_(1/2))₃AlC₂ (50% HF 69 hours at RT).

FIG. 16 shows SEM micrographs of chemically exfoliated(V_(1/2)Cr_(1/2))₃AlC₂ (50% HF 69 hours at RT).

FIG. 17 shows SEM micrographs and EDX analytical results of chemicallyexfoliated Ti₃AlC₂ (10% HF 2 hours at 65° C.).

FIGS. 18A and B show TEM micrographs of chemically exfoliated Ti₃AlC₂(50% HF 22 hours at RT). FIG. 18B is an enlargement of bottom micrographof FIG. 18A.

FIG. 19 shows XRD spectra for samples of chemically exfoliated Ti₃AlC₂,generated as a function of temperature in 50 wt % HF for 2 hours.

FIG. 20 shows XRD spectra for samples of chemically exfoliated Ti₃AlC₂,generated as a function of time in 50 wt % HF at room temperature.

FIG. 21 shows resistivity for various chemically exfoliated Ti₃AlC₂,generated as a function of time, in 50% HF at ambient temperature.

FIG. 22A shows XRD spectra for samples of chemically exfoliated Ti₃AlC₂,generated as a function of initial particle size of the MAX phaseprecursor. FIG. 22B shows calibration curve derived from data from FIG.22A. The materials were held for 2 hours in 50 wt % HF at roomtemperature

FIG. 23 shows XRD spectra forming the basis for a calibration curve(inset) of chemically exfoliated Ti₃AlC₂, generated as a function ofcomposition.

FIG. 24 shows TGA graphs for two samples of Ti₃C₂(OH)_(x)(F)_(y)(alternatively, Ti₃C₂T_(s)) prepared at two different drying conditions.Upper sample 1.79 mg (4.76%), ramped 50.00° C./min to 30° C.; lowersample 3.44 mg (8.57%), ramped 5.00° C./min to 500° C.; both samplestested under N2 @ 60.0 mL/min.

FIG. 25 shows exemplary experimental parameters for tests to evaluatethe electrochemical exfoliation of MAX phase materials; see Example 9.

FIG. 26 shows a series of Raman spectra for samples obtained in testsdescribed in Example 8. Curves (a) and (b) are the spectra of theexfoliated material obtained after the electrochemical aniodicpolarization treatment of the Ti₂SnC MAX phase in 12 M HCl. Curves (c)through (e) are the initial spectra of Ti₂SnC.

FIG. 27( a) shows the XRD data of the exfoliated material obtained afterthe electrochemical aniodic polarization treatment of the Ti₂SnC MAXphase in 12 M HCl. FIG. 27( b) shows the XRD data of the initial Ti₂SnCmaterial. FIG. 27( c) shows the XRD data of the Ti₂SnC material treatedin Ar as 1250° C. FIG. 27( d) is a simulated XRD pattern for the Ti₂SnCsystem. Note: the asterisks and crossed boxes mark the characteristicpeak positions of Si_(pc) and TiC, respectively.

FIGS. 28(A), (B), and (C) show SEM and EDX/EDS data of the exfoliatedmaterial obtained after the electrochemical aniodic polarizationtreatment of the Ti₂SnC MAX phase in 12 M HCl. The circles represent theapproximate area subjected to EDX/EDS analysis.

FIGS. 29(A) and 29(B) shows the results of electrochemical testingdescribed in Example 10. The labels “Exfoliated” and “Not Exfoliated”refer to the samples used in the testing corresponding to exfoliated andnot exfoliated particles of Ti₃AlC₂.

FIG. 30A shows an SEM image of exfoliated Ti₂CO_(x) produced by HFtreatment of Ti₂AlC, and FIG. 30B shows N₂ adsorption-desorptionisotherms of the material shown in FIG. 30A, circles refer to adsorptionand squares refer to desorption. The calculated SSA is approximately 23m²·g⁻¹.

FIG. 31(A) shows cyclic voltammetry curves of exfoliated Ti₂C(alternatively, Ti₂CT_(s)) at a constant scan rate of 0.2 mV·s⁻¹. Thesolid arrows refer to main peaks positions during lithiation anddelithiation cycles. FIG. 31(B) provides the galvanostaticcharge/discharge curves at a C/10 rate. FIG. 31(C) shows specificlithiation (circles in the figure) and delithiation (squares in thefigure) capacities (per mass of active material) vs. cycle number atdifferent rates. The inset in FIG. 31(C) is a zoom of the first 20cycles.

FIG. 32 shows a schematic illustration of the intercalation of cationsbetween Ti₃C₂T_(s) layers. The interlayer spacing d increases after ionintercalation.

FIG. 33A-C shows X-ray diffractions patterns of various salts: FIG. 33Afor compounds which form basic solutions when dissolved in water, FIG.33B for sulfate salts which form nearly neutral solutions when dissolvedin water and, FIG. 33C for Na-salts with different organic anions. Inall figures the location of the Ti₃C₂T_(s) peak before immersion in thesalt solutions is depicted by dashed vertical line. In all cases, thec-lattice parameter increases by the values shown and ranged from a highof 5 Å to a low of 0.7 Å.

FIG. 34 shows a schematic of the electrochemical testing set-updescribed in Example 13.2: 3-electrode Swagelok cell, with multilayerTi₃C₂T_(s) as working electrode (cubic elements).

FIG. 35 shows electrochemical performance of Ti₃C₂T_(s)-basedsupercapacitors in various aqueous electrolytes. FIG. 35A shows profilesin NaOH—, KOH—, and LiOH-containing solutions at 20 mV/s. FIG. 35B showsthe CV profiles in K₂SO₄, Al₂(SO₄)₃, and Al(NO₃)₃ solutions at 20 mV/s.FIG. 35C provides a summary of rate performances in different aqueouselectrolytes.

FIG. 36 shows Nyquist plot of the Ti₃C₂T_(s) electrodes in NaOH, KOH,LiOH (FIG. 36A) and K₂SO₄, Al₂(SO₄)₃, and Al(NO₃)₃ solutions (FIG. 36B).

FIG. 37 shows cyclic voltammetry sweeps of the Ti₃C₂T_(s) in differentpotential windows (can rate 10 mV/s) in 1 M KOH.

FIG. 38 shows cyclic voltammograms of Ti₃C₂T_(s) in 1 M MgSO4electrolyte collected after 0 h, 1 h, 6 h, 18 h, and 48 h of testing.Inset is a schematic illustration of the electrochemically inducedcationintercalation between the layers of MXene

FIG. 39 shows cyclic voltammograms of Ti₃C₂T_(s) in 1 M MgSO₄electrolyte collected during different cycling regimes: (1) chemicalintercalation, screening cycling in-between; (2) continuous cycling at 5mVs; holding at −0.7 V, screening cycling in-between; collected after1.5 hours (FIG. 39A) and 7.5 hours (FIG. 39B) from the beginning of theexperiment.

FIG. 40A-B shows results of electrochemical in situ x-ray diffractionstudy of multilayer exfoliated Ti₃C₂T_(s). FIG. 40A shows Ti₃C₂T_(s) in1 M KOH solution; FIG. 40B shows Ti₃C₂T_(s) in 1 M MgSO₄ solution.Vertical dashed lines indicate the original position of the (0002) peakof the Ti₃C₂T_(s) electrodes before mounting in a cell. Inclined arrowsshow the direction of the (0002) peak shift. Insets illustrate cyclingdirection and concomitant changes in the c lattice parameters duringcycling. In both KOH and MgSO₄ electrolytes, shrinkage during cathodicpolarization was observed.

FIG. 41 shows results of electrochemical in-situ X-ray diffraction studyof Ti₃C₂T_(s) in 3 M sodium acetate. Arrows indicate the direction ofthe (0002) peak shift. See Example 13.2.

FIG. 42A shows cyclic voltammetry data of Ti₃C₂T_(s) paper in KOHelectrolyte. FIG. 42B shows EISdata in KOH for Ti₃C₂T_(s) electrode(KOH, solid symbols) and Ti₃C₂T_(s) paper (p-KOH). FIG. 42C shows rateperformance of the Ti₃C₂T_(s) paper (open symbols) versus multilayerexfoliated Ti₃C₂T_(s) electrode (solid symbols) in KOH—, MgSO₄—, andNaOAc-containing electrolytes. FIG. 42D shows capacitance retention testof Ti₃C₂T_(s) paper in KOH. Inset: Galvanostatic cycling data collectedat 1 A/g.

FIG. 43 shows data for electrochemistry of Ti₃C₂T_(s) paper. FIG. 43Ashows a cyclic voltammogram in 1M MgSO₄ electrolyte. Capacitanceretention test of MXene paper in 1 M MgSO₄ are provided in FIG. 43B and3 M NaOAc (FIG. 43C) Insets in FIG. 43B and FIG. 43C show results ofgalvanostatic cycling data of Ti₃C₂T_(s) paper in MgSO₄ and NaOAc,respectively.

FIG. 44A-B shows Ti₃C₂T_(s) paper cell performance in K₂SO₄. FIG. 44Ashows cyclic voltammetry data at different scan rates; FIG. 44B showscapacitance retention test. Inset: gavlanostatic cycling data collectedat 1 A/g.

FIG. 45A-C shows evidence of MXene intercalation. FIG. 45A shows aschematic representation of the synthesis and intercalation of MXene. Toproduce MXene, the Al layer was removed from the corresponding MAX phasein aqueous HF solution resulting in OH terminated MXene layers. ThenMXene was treated with an intercalant (urea is shown here as an example)yielding MXene intercalation compound. FIG. 45B, XRD patterns of MXene:(i) as-received, before any treatment, (ii) after HM treatment, and thenwashed with ethanol, and (iii) after HM in DMF treatment, washed withDMF, dried at different conditions: a whole range diffractograms. FIG.45C is an expanded image of FIG. 45B but zoomed on the (002) peak in5-12° 2θ range. Intercalation was performed at 80° C. for 24 h. TheMXene powder used for intercalation was dried at 100° C. for 22 h. SeeExample 14.

FIG. 46 shows XRD patterns of MXene: (i) before any treatment, (ii)after HM treatment in DMF at 80° C., (iii) after HM treatment at 80° C.;and after HM treatment in DMF at 80° C. (iv) and at relevant temperature(v). Patterns i-iii: initial MXene was dried at 100° C. for 22 h beforeintercalation. Patterns iv-v: as-received wet MXene was used as aninitial material. The resulting powders were washed with DMF afterintercalation. See Example 14.

FIG. 47A-B shows XPS spectra of MXene intercalated with HM at 80° C. for24 h (FIG. 47A) and with HM and DMF at 80° C. for 24 h (FIG. 47B). Bothinsets showed N1s peaks for corresponding samples.

FIG. 48 shows images from electron microscopy analyses, including SEMimages before (FIG. 48A) and after (FIG. 48B) intercalation of MXenewith HM and DMF (24 h at 80° C.), respectively; TEM (FIG. 48C) and HRTEM(FIG. 48D) images with corresponding SAED pattern as inset of MXenebefore intercalation; TEM image (FIG. 48E) and SAED pattern (FIG. 48F)of intercalated MXene.

FIG. 49 shows XRD patterns of MXene: (i) before any treatment, (ii)after DMSO treatment taken 30 min and 3 weeks after drying in adesiccator at RT, (iii) after urea treatment.

FIG. 50 shows XRD patterns of Ti₃CN-based MXene (i) before and after HMtreatment and TiNbC-based MXene (i) before and after HM treatment.

FIG. 51 shows schematic diagram of steps used to produce epitaxial MXenefilms. FIG. 51A is a cartoon representation of magnetron sputtering ofTi, Al and C forming a few-nanometer TiC incubation layer on a 0001sapphire substrate, followed by the deposition of Ti₃AlC₂; FIG. 51Bshows a STEM image of the first two Ti₃C₂T_(s) layers after applyingWiener filter; scale bar is equal to 1 nm.

FIG. 52A-E provide spectra for phase and chemical analysis: FIG. 52Aprovides XRD patterns of as-deposited—60 nm nominal thickness—Ti₃AlC₂thin films (I), Ti₃C₂T_(s) after etching in 50% HF for 2 h 40 min (II),and Ti₃C₂T_(s)-IC after etching in 1M NH₄HF₂ for 11 h (III). XPS spectraof, Ti 2p (FIG. 52B), C 1s (FIG. 52C), and Al 2p (FIG. 52D), forTi₃AlC₂, Ti₃C₂T_(s) and Ti₃C₂T_(s)-IC thin films, respectively. Thevertical lines in b and c indicates the positions of Ti (3/2p and 1/2p)and C (1s) binding energies in TiC, respectively. FIG. 52E providesdeconvolution of high resolution XPS spectra for N 1s region forTi₃C₂T_(s)-IC, best fitted by symmetric Gaussian-Lorentzian curvesresting on a Shirley background. The two components correspond to (NH₄⁺¹) and (NH₃).

FIG. 53A-F shows cross-sectional STEM images of Ti₃AlC₂ films before andafter etching. STEM images of Ti₃AlC₂ (FIG. 53A), Ti₃C₂T_(s), (FIG. 53B)and Ti₃C₂T_(s)-IC films (FIG. 53C) (60 nm nominal thickness) grown on asapphire substrate with a TiC incubation layer. Insets show SAED of thefilm and the substrate. The subscripts A and T correspond to Al₂O₃ andTi₃AlC₂, respectively. High-resolution STEM images of Ti₃AlC₂ (FIG.53D), Ti₃C₂T_(s) (FIG. 53E), and Ti₃C₂T_(s)-IC films (FIG. 53F), alongthe [11 20] zone axis. Scale bars for low resolution (FIG. 55A-C), andhigh-resolution (FIG. 53D-F) images correspond to 5 nm and 1 nm,respectively.

FIG. 54-B illustrate the optical behavior of non-etched and etchedTi₃AlC₂ thin films; FIG. 54A provides transmittance spectra and visualimage (on right) for, (I) Ti₃AlC₂, (II) Ti₃C₂T_(s) and (III)Ti₃C₂T_(s)-IC films of 15 nm nominal thickness. The film are ca. 1×1 cm²in area; FIG. 54B provides light absorbance at wavelengths of 240 and800 nm vs. thickness of Ti₃C₂T_(s) and Ti₃C₂T_(s)-IC films.

FIG. 55A-C shows the dependence of the electrical behavior of Ti₃AlC₂,Ti₃C₂T_(s) and Ti₃C₂T_(s)-IC films on temperature and magnetic field.FIG. 55A provides data for resistivity vs. temperature for Ti₃AlC₂,Ti₃C₂T_(s) and Ti₃C₂—IC films of 20 nm nominal thickness. FIG. 55Bprovides data for resistivity vs. temperature for Ti₃C₂T_(s) of 20 nmnominal thickness. Inset shows fitting of resistivity, over thetemperature range of 2 to 74 K, to the weak localization model (ρ˜ln T)and, FIG. 55C compares normalized magnetoresistance curves forTi₃C₂T_(s) of 28 nm nominal thickness at various temperatures rangingfrom 2.5 to 200 K. RH=0 refers to the film resistance in the absence ofapplied magnetic field.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

The present invention may be understood more readily by reference to thefollowing detailed description taken in connection with the accompanyingFigures and Examples, which form a part of this disclosure. It is to beunderstood that this invention is not limited to the specific products,methods, conditions or parameters described and/or shown herein, andthat the terminology used herein is for the purpose of describingparticular embodiments by way of example only and is not intended to belimiting of any claimed invention. Similarly, unless specificallyotherwise stated, any description as to a possible mechanism or mode ofaction or reason for improvement is meant to be illustrative only, andthe invention herein is not to be constrained by the correctness orincorrectness of any such suggested mechanism or mode of action orreason for improvement. Throughout this text, it is recognized that thedescriptions refer both to compositions and to the articles and devicesderived therefrom, as well as the methods of manufacture and use.

In the present disclosure the singular forms “a,” “an,” and “the”include the plural reference, and reference to a particular numericalvalue includes at least that particular value, unless the contextclearly indicates otherwise. Thus, for example, a reference to “amaterial” is a reference to at least one of such materials andequivalents thereof known to those skilled in the art, and so forth.

When values are expressed as approximations by use of the antecedent“about,” it will be understood that the particular value forms anotherembodiment. In general, use of the term “about” indicates approximationsthat can vary depending on the desired properties sought to be obtainedby the disclosed subject matter and is to be interpreted in the specificcontext in which it is used, based on its function, and the personskilled in the art will be able to interpret it as such. In some cases,the number of significant figures used for a particular value may be onenon-limiting method of determining the extent of the word “about.” Inother cases, the gradations used in a series of values may be used todetermine the intended range available to the term “about” for eachvalue. Where present, all ranges are inclusive and combinable. That is,reference to values stated in ranges includes each and every valuewithin that range.

It is to be appreciated that certain features of the invention whichare, for clarity, described herein in the context of separateembodiments, may also be provided in combination in a single embodiment.Conversely, various features of the invention that are, for brevity,described in the context of a single embodiment, may also be providedseparately or in any subcombination. Finally, while an embodiment may bedescribed as part of a series of steps or part of a more generalcomposition or structure, each said step may also be considered anindependent embodiment in itself.

Various embodiments of this invention provide for crystallinecompositions comprising at least one layer having first and secondsurfaces, each layer comprising a substantially two-dimensional array ofcrystal cells; each crystal cell is an ordered array of atoms having anempirical formula of M_(n+1)X_(n), such that each X is positioned withinan octahedral array of M; wherein M is at least one Group IIIB, IVB, VB,or VIB metal; wherein each X is C or N (i.e., stoichiometricallyX═C_(x)N_(y), including where x+y=1); and n=1, 2, or 3. In someembodiments, these compositions comprise a plurality of layers. Otherembodiments provide for stacked assemblies of such layers. Collectively,such compositions are referred to herein as “MXene,” “MXenecompositions,” or “MXene materials.” Additionally, these terms “MXene,”“MXene compositions,” or “MXene materials” also refer to thosecompositions derived by the chemical exfoliation of MAX phase materials,whether these compositions are present as free-standing 2-dimensional orstacked assemblies (as described further below). FIG. 1 provides arepresentation of the crystal cells of various M_(n+1)X_(n) (where n=1,2, or 3) frameworks, presented however, in the context of correspondingMAX-phase materials (see also below). In various embodiments, each X ispositioned within an octahedral array of M.

Analogous to other so-called two-dimensional, atomically-scaled layeredsolid materials, such as graphene or hexagonal BN, these MXenecrystalline compositions may be free-standing or be present in stackedcompositions. As used herein, the term “free standing” refers toindividual layers wherein the adjacent composite crystal layers are notbonded to one another by covalent bonds or connected by metal-latticebonds, but may be joined by intervening hydrogen (or even weaker)bonding, such that each such layer can be physically manipulated. Seee.g., FIGS. 2 and 3. However, this term does not preclude the depositionof these layers or stacked layers on substrates or within polymercompositions (see also below).

The term “crystalline compositions comprising at least one layer havingfirst and second surfaces, each layer comprising a substantiallytwo-dimensional array of crystal cells” refers to the unique characterof these materials. For purposes of visualization, the two-dimensionalarray of crystal cells may be viewed as an array of cells extending inan x-y plane, with the z-axis defining the thickness of the composition,without any restrictions as to the absolute orientation of that plane oraxes. It is preferred that the at least one layer having first andsecond surface contain but a single two-dimensional array of crystalcells (that is, the z-dimension is defined by the dimension ofapproximately one crystal cell), such that the planar surfaces of saidcell array defines the surface of the layer, it should be appreciatedthat real compositions may contain portions having more than singlecrystal cell thicknesses.

That is, as used herein, “a substantially two dimensional array ofcrystal cells” refers to an array which preferably includes a lateral(in x-y dimension) array of crystals having a thickness of a single cell(e.g., corresponding to the M₂X, M₃X₂, or M₄X₃ cells as depicted in FIG.1), such that the top and bottom surfaces of the array are available forchemical modification.

It should also be appreciated that, analogous to graphene or hexagonalBN compositions, this description of a planar or two-dimensional arrayshould not be interpreted to describe a necessarily flat structure;rather such compositions may also take the form of a curved orundulating plane, a scroll, or a cylinder or tube (e.g., analogous tothe structure of a carbon or BN nanotube).

In certain embodiments, the compositions may contain C or N atoms, or amixture thereof, but in any case, these atoms are positioned within anoctahedral or pseudo-octahedral array of M atoms, reminiscent of thepositioning of the carbon or nitrogen atom within MAX-phase materials.While not necessarily being bound to the scientific accuracy of thisstatement, this arrangement appears to protect the C and/or N atoms fromexternal chemical attack, while at the same time providing a degree ofstructural strength to the 2-dimensional layers.

Given the difficulties in obtaining crystallographic evidence as to thecrystallinity of materials having such few layers (e.g., less than about5 cell layers), owing to the reduced level or lack of constructiveinterference of such few layers, these materials may be characterized bymeasuring the thickness of the individual layers (measured, for example,by Transmission Electron Micrography or atomic force microscopy).Depending on the particular empirical formula of the given material, thethickness of a given single cell layer will be on the order of about 0.2to about 0.3 nm (preferably about 0.25 nm) for M₂X compositions, about0.3 to about 0.7 nm (preferably about 0.5 nm) for M₃X₂ compositions, andabout 0.6 to about 0.9 nm (preferably about 0.75 nm) for M₄X₃compositions. As described more fully below, one method of preparingthese compositions is to react a precursor MAX phase material so as toremove the labile A-phase, and exfoliating the resulting structure. Inthese cases, it is so generally observed that the crystallinity of theresulting MXene framework, which existed in the original MAX phasestructure, is sufficiently robust as to be retained during thepreparation process, so that the thickness measurements by themselvescan be used to characterize the materials, even in the absence ofcrystallographic analysis.

These MXene materials (even individual or exfoliated layers) can also becharacterized by measuring the X-ray diffraction (XRD) spectra of(optionally cold pressed) stacked layers (see, e.g., Example 2, FIG. 4(a) and Example 4, FIG. 7 below). That is, such stacking provides asample of sufficient thickness (number of layers) to allow forsufficient constructive interference so as to provide for a measurableXRD pattern to be obtained. One distinguishing feature of XRD patternsthus generated is the presence of peaks at 2θ of ca. 5-7° (i.e., betweenabout 4.5° and about 9.5° when Cu K_(α) radiation is used),corresponding to the d-spacing (thickness) of the individual layers(including the surface coatings of each layer) and lower than the (002)peaks of the corresponding MAX phase materials. That this MXene peakoccurs at lower 2θ values, reflecting higher d-spacings of the layers,than the corresponding (002) plane in a corresponding MAX phase materialis consistent with the greater spacing of the crystal cells of the twomaterials in the former relative to the latter (e.g., referring to FIG.3, the individual layers of the Ti₃C₂ in FIG. 3B are spaced furtherapart than the corresponding layers in FIG. 3A).

As described herein, the terms “M” or “M atoms,” “M elements,” or “Mmetals” refers to one or more members of the Groups IIIB, IVB, VB, orVIB or (aka) Groups 3-6 of the periodic table, either alone or incombination, said members including Sc, Y, Lu, Ti, Zr, Hf, V, Nb, Ta,Cr, Mo, and W. The terms “M” or “M atoms,” “M elements,” or “M metals”may also include Mn. In preferred embodiments, the transition metal isone or more of Sc, Ti, Zr, Hf, V, Nb, Ta, Cr, and/or Mo. In otherpreferred embodiments, the transition metal is one or more of Ti, Zr, V,Cr, Mo, Nb, and/or Ta. In even more preferred embodiments, thetransition metal is Ti, Ta, Mo, Nb, V, and/or Cr.

The empirical formula M_(n+1)X_(n), wherein X is C, N, or a combinationthereof, and n=1, 2, or 3 gives rise to a number of possiblecomposition. For example, and while not intending to be limited to thislist, exemplary compositions when n=1 includes those wherein theempirical formula of the crystalline phase is Sc₂C, Sc₂N, Ti₂C, Ti₂N,Mo₂C, V₂C, V₂N, Cr₂C, Cr₂N, Zr₂C, Zr₂N, Nb₂C, Nb₂N, Hf₂C, and Hf₂N.Similarly, non-limiting exemplary compositions when n=2 includes thosewherein the empirical formula of the crystalline phase is Ti₃C₂, Ti₃N₂,V₃C₂, V₃C₂, Ta₃C₂, and Ta₃N₂ and when n=3 includes those wherein theempirical formula is Ti₄C₃, Ti₄N₃, V₄C₃, V₄N₃, Ta₄C₃ and Ta₄N₃.Especially important independent embodiments include those where Mcomprises at least one Group IVB element, for example Ti, Zr, or Hf andthose where M comprises at least one Group V elements, for example V,Nb, or Ta. More preferred independent embodiments include those where Mis Ti or Ta, especially structures wherein the empirical formula of thecrystalline phase is Ti₂C, Ti₂N, Ti₃C₂, Ti₃N₂, Ti₄C₃, or Ti₄N₃, orTa₃C₂, Ta₃N₂, Ta₄C₃ or Ta₄N₃, especially Ti₂C or Ta₄C₃.

The range of compositions available can be seen as extending evenfurther when one considers that each M-atom position within the overallM_(n+1)X_(n) matrix can be represented by more than one element. Thatis, one or more type of M-atom can occupy each M-positions within therespective matrices. In certain exemplary non-limiting examples, thesecan be (M^(A) _(x)M^(B) _(y))₂C or (M^(A) _(x)M^(B) _(y))₂N, (M^(A)_(x)M^(B) _(y))₃C₂ or (M^(A) _(x)M^(B) _(y))₃C₂, or (M^(A) _(x)M^(B)_(y))₄C₃ or (M^(A) _(x)M^(B) _(y))₄C₃, where M^(A) and M^(B) areindependently members of the same group, and x+y=1. For example, in butone non-limiting example, such a composition can be(V_(1/2))Cr_(1/2))₃C₂. In the same way, one or more type of X-atom canoccupy each X-position within the matrices, for example solid solutionsof the formulae M_(n+1)(C_(x)N_(y))_(n), or (M^(A) _(x)M^(B)_(y))_(n+1)(C_(x)N_(y))_(n).

In various embodiments, the composition's layer has first and secondsurfaces which are capable of being physically or chemicallyinterrogated or modified. This feature distinguishes these compositionsfrom sputtered matrices or so-called MAX phase compositions. While itmay be possible to describe sputtered matrices or MAX phase compositionsas containing two-dimensional arrays of crystal cells, in each casethese are embedded within vertically integrated and practically bound toother layers within the respective matrices (e.g., in the case ofsputtered matrices, to other neighboring sputtered layers or thesubstrate; in the case of MAX-phase compositions, to interleaved A-groupelement arrays), either by covalent, metallic, or lattice bonds, andwhich cannot be separately accessed. By contrast, in various embodimentsof the present compositions, each layer has two available or accessiblesurfaces sandwiching each substantially two-dimensional array of crystalcells, each of which surfaces can be accessed for physical or chemicalinterrogation or modification.

It is important to note that, as prepared, the 2D MXene surfaces are notM-terminated (e.g., Ti-terminated), but primarily covered by oxide, OH,F groups or some combination thereof. For example in the case of a MXeneof nominal composition Ti₃C₂ (e.g., derived from MAX phase Ti₃AlC₂) infact is probably better represented by a formula such asTi₃C₂(OH)_(x)O_(y)F_(z). However, since the exact surface compositionmay not be known with certainty and can vary from sample to sample, andfor the sake of brevity, herein, such MXene compositions of this sort(e.g., such as derived from Ti₃AlC₂) may be referred to as Ti₃C₂(OH)_(x)O_(y)F_(z), Ti₃C₂, or Ti₃C₂T_(s) (where T_(s) refers to “surfaceterminations”), or more generally M_(n+1)X_(n)T_(s), the latter termsbeing useful to replace the more cumbersome former term, in a mannersimilar to the use of a general name “graphene oxide” for oxidizedgraphene, which has a variety of oxygen-containing groups.

Having said this, the ability to functionalize the surfaces of thelayers of the present invention to provide enrichment of a particularfunctional group provides a considerable synthetic and structuralflexibility. Because of the arrangement of the M atoms within theM_(n+1)X_(n) framework, wherein each X is positioned within anoctahedral array of M atoms, the “unfunctionalized” surface compriseslargely M atoms. For example, in the absence of imperfections, asubstantially planar array of crystal cells having an empirical formulaTi₃C₂ will provide or present external surfaces comprising a planararray of Ti atoms (see, e.g., FIG. 3). At the same time, owing to thechemical reactivity of Ti (or any of the M atoms), these surfaces willbe coated with one or more organic or inorganic moieties, generallycomprising heteroatoms or having heteroatom linking groups.

For example, in certain embodiments, at least one of the surfaces arecoated with a coating comprising H, N, O, or S atoms, for example, ahydride, oxide, sub-oxide, nitride, sub-nitride, sulfide, orsub-sulfide. In preferred embodiments, the coating comprises a hydratedor anhydrous oxide, a sub-oxide, or some combination thereof. As usedherein the terms “sub-oxide,” “sub-nitride,” or “sub-sulfide” isintended to connote a composition containing an amount reflecting asub-stoichiometric or a mixed oxidation state of the M metal at thesurface of oxide, nitride, or sulfide, respectively. For example,various forms of titania are known to exist as TiO_(x), where x can beless than 2. Accordingly, the surfaces of the present invention may alsocontain oxides, nitrides, or sulfides in similar sub-stoichiometric ormixed oxidation state amounts.

In other embodiments, at least one surface is coated with a coatinghaving a pendant moiety which is linked to the surface by an N, O, or Satom (e.g., an M—N, M—O, or M—S bond, respectively). Such surfacecoatings then may comprise at least one hydroxide, alkoxide,carboxylate, amine, amide, or thiol. These pendants may contain organicmoieties, including saturated, unsaturated, and/or aromatic moieties.These organic moieties may optionally include heteroatoms, be linear orbranched, and/or may contain one or more functional groups, for exampleamines and derivatives therefrom, (thio)carboxylic acids and derivativestherefrom, hydroxy or ether groups, and/or thiol groups. The moietiesand/or optionally available functional groups may exist in their neutralor ionized state.

In other embodiments, the coating of at least one surface comprises atleast one halide, for example F, Cl, Br, or I, preferably F. As usedherein, the terms “halide” and, e.g., “fluoride” are intended to reflectthe presence of metal-halogen or metal-fluorine bonds, respectively,without regard to the specific nominal charge on the halogen orfluorine.

The skilled artisan will be able to interchange the pendant groups bymethods known in the art. Without the need for an exhaustive delineationof such methods, in one non-limiting example, a hydroxy or alkoxysurface may be prepared by providing an excess hydroxide or alkoxide soas to displace the halide from an initially presented M-halide surfaceor so as to hydrate or alkoxylate a metal oxide or sub-oxide surface.Similarly, an originally presented M-hydroxide surface may be convertedto oxide or sub-oxide surface by application of heat or otherdehydrating conditions. Nitrogen and sulfur surfaces may be analogouslyinterconverted by methods known in the art for making such conversions.Similarly, hydrides may be prepared by exposing precursors to reducingconditions, either electrolytically or by contacting with reducingagents such as hydrides (e.g., NaBH₄), hydrogen gas, or ammonia.

In certain embodiments, the compositions may be electrically conductingor semiconducting.

In certain embodiments, the compositions of the present inventioncomprises at least one individual layer having first and secondsurfaces, each layer comprising a substantially two-dimensional array ofcrystal cells having an empirical formula Ti₃C₂, with at least onesurface coated with a coating comprising a hydroxide, an oxide, asub-oxide, or a combination thereof, and so optionally represented asTi₃C₂T_(s). In other embodiments, the coating comprises fluorine orfluoride.

In other embodiments, the crystalline composition comprises at least oneindividual layer having first and second surfaces, each layer comprisinga substantially two-dimensional array of crystal cells having anempirical formula Ta₄C₃, with at least one surface coated with a coatingcomprising a hydroxide, an oxide, a sub-oxide, or a combination thereof,and so represented as Ta₄C₃T_(s).

In still other embodiments, the crystalline composition comprises atleast one individual layer having first and second surfaces, each layercomprising a substantially two-dimensional array of crystal cells havingan empirical formula (Cr_(x)V_(x))₃C₂ (including where x=y=½) with atleast one surface coated with a coating comprising a hydroxide, anoxide, a sub-oxide, or a combination thereof.

As described above, certain additional embodiments provide MXenecompositions which exhibit conductive or semi-conductive behavior, aswell as those electronic devices (e.g., transistors, where the use ofgraphene and MoS₂ has been successfully demonstrated) which incorporatesuch compositions so as to take advantage of this property. Further, itis shown that variations in the nature of the surface coating effectsthat behavior, as shown by density functional theory (DFT) calculations(methods described in Example 1, below) (FIG. 5( g)). For example, thecalculated band structure of a single Ti₃C₂ layer resembles a typicalsemi-metal with a finite density of states at the Fermi level. Indeed,the resistivity of the thin disk shown in FIG. 4( e) is estimated toabout an order of magnitude higher than the same disc made withunreacted Ti₂AlC powders, which translates to a resistivity of ca. 0.03μΩm. By contrast, when terminated with OH and F groups, the bandstructure has a semiconducting character with a clear separation betweenvalence and conduction bands by 0.05 eV and 0.1 eV, respectively (FIG.5( g)), thereby supporting the conclusion that it is possible to tunethe electronic structure of exfoliated MAX layers—or MXenecompositions—by varying the functional groups. Such furthermodifications of the functional groups themselves may provide additionalflexibility in this regard.

In certain embodiments, MXene films or papers are sufficiently thin asto be transparent (see, e.g., Example 16), while maintaining surfaceconductivity. Optical transparencies as high as 90% have been obtained,though in certain embodiments, such MXene films or papers may exhibitoptical transparencies (i.e., at least one wavelength in a range ofabout 250 nm to about 850 nm) in a range of from about 0% to about 95%or higher, from about 50% to about 95%, from about 70% to about 95%, orfrom about 70% to about 90%. Such thin films may be prepared bydelaminating epitaxially grown thin films, either as-prepared orintercalated with one or more materials as described herein.

Additional embodiments provide for the use or incorporation of MXenecompositions into other materials, or the incorporation of othermaterials within them. For example, various embodiments provide polymercomposites into which a MXene composition is incorporated. Moreparticularly, further embodiments provide polymer composite compositionswherein the MXene compositions comprises between amounts in the range ofabout 0.1 wt % to about 50 wt %, relative to the combined weight of thepolymer and MXene composition. Still other embodiments provide that theMXene composition is present in a range whose lower amount is about 0.1,about 1, about 2, about 5, or about 10 wt % and the upper amount isabout 50 wt %, about 40 wt %, about 30 wt %, about 20 wt %, about 10 wt%, or about 5 wt %, relative to the combined weight of the polymer andthe MXene composition comprising a polymer.

The polymer composite may be comprised of organic polymers, morespecifically thermoset or thermoplastic polymers or polymer resins,elastomers, or mixtures thereof. Various embodiments include thosewherein the polymer or polymer resin contains an aromatic orheteroaromatic moiety, for example, phenyl, biphenyl, pyridinyl,bipyridinyl, naphthyl, pyrimidinyl, including derivative amides oresters of terephthalic acid or naphthalic acid. Other embodimentsprovide that the polymer or polymer resin comprises polyester,polyamide, polyethylene, polypropylene, polyethylenenaphthalate (PEN),polyethylene terephthalate (PET), polybutylene terephthalate (PBT),polyether etherketone (PEEK), polyamide, polyaryletherketone (PAEK),polyethersulfone (PES), polyethylenenimine (PEI), poly (p-phenylenesulfide) (PPS), polyvinyl chloride (PVC), fluorinated or perfluorinatedpolymer (such as a polytetrafluoroethylene (PTFE or TEFLON®),polyvinylidene difluoride (PVDF), a polyvinyl fluoride (PVF or TEDLAR®))(TEFLON® and TEDLAR® are registered trademarks of the E.I. DuPont deNemours Company, of Wilmington, Del.)

It is believed that the planar nature of MXene layers may be well suitedto organizing themselves in those anisotropic polymers, for examplehaving planar moieties, e.g., aromatic moieties, especially when (butnot only when) these planar organic moieties are directionally orientedto be parallel in a polymer composite composition. Such embodimentsinclude the inclusion of MXene compositions into liquid crystalpolymers. Moreover, the ability to produce MXene compositions havingboth hydrophobic and hydrophilic pendants provides for compatibilitywith a wide-ranging variety of polymer materials.

Additional embodiments of the present invention provide polymercomposites, including those wherein the polymer composite is in a formhaving a planar configuration—for example, a film, sheet, orribbon—comprising a MXene layer or multilayer composition. Still furtherembodiments provide such polymer composites wherein the two-dimensionalcrystal layers of the MXene materials are aligned or substantiallyaligned with the plane of a polymer composite film, sheet, or ribbon,especially when the organic polymers are oriented in the plane of thatfilm, sheet, or ribbon.

The large elastic moduli predicted by ab initio simulation, and thepossibility of varying their surface chemistries (beyond thoseexemplified herein, which are terminated by hydroxyl and/or fluorinegroups) render these nanosheets attractive as polymer composite fillers.For example, the elastic modulus of a single, exfoliated Ti₃C₂(OH)₂layer, along the basal plane, is calculated to be around 300 GPa, whichis within the typical range of transition metal carbides andsignificantly higher than most oxides and clays (see, e.g., P. H.Nadeau, Applied Clay Science 1987, 2, 83, which is incorporated byreference herein in its entirety). And while the 300 GPa value is lowerthan that of graphene (e.g., as described in S. Stankovich, et al.,Nature 2006, 442, 282, which is incorporated by reference herein in itsentirety), the ability to match the character of the MXene layeredmaterials with that of the polymer matrix, as described above, isexpected to ensure better bonding to and better dispersion in polymermatrices when these MXene layers are to be used as reinforcements inpolymer composites. It is also important to note here that thefunctionalized Ti₃C₂ sheets described herein were much more stable thangraphene sheets under the 200 kV electron beam in the TEM.

Accordingly, still further embodiments provide that the MXenecomposition-filled composite polymers, especially when these polymercomposites have a planar configuration, such as that of film, sheet, orribbon, especially an oriented film, sheet, or ribbon, exhibit aflexural strength (bending rigidity) and/or stiffness than that of thecorresponding film, sheet, or ribbon of the same polymer without theMXene composition. In some embodiments, this greater flexural strengthand/or stiffness is independently at least 5%, at least 10%, or at least25% higher than the flexural strength or toughness than that exhibitedby an otherwise equivalent, but unfilled material.

Thus far, the compositions have been described in terms of havingindividual layers having first and second surfaces, each layercomprising a substantially two-dimensional array of crystal cells.However, additional embodiments provide for stacked assemblies of atleast two layers having first and second surfaces, each layer comprisinga substantially two-dimensional array of crystal cells, each crystalcell having the empirical formula of M_(n+1)X_(n), such that each X ispositioned within an octahedral array of M; wherein M is a Group IIIB,IVB, VB, or VIB metal or Mn; each X is C or N; and n=1, 2, or 3; andwherein the layers are characterized as having an average surface areaand interlayer distance.

In various embodiments of these stacked assemblies, each layer mayretain the characteristics as described above, but be held in place oredge-wise connected such that the assembly has up to about 50 layers ofcrystal layers. In various embodiments, these number of crystal layersin these assemblies may be described as having a range having a lowerend of 2, about 5, about 10, about 15, or about 20 and an upper range ofabout 50, about 40, about 30, about 25, about 20, and about 10, withexemplary ranges of 2 to about 50, 2 to about 25, 2 to about 20, about 5to about 50, about 5 to about 25, about 5 to about 20, about 10 to about50, about 10 to about 25, about 10 to about 20, about 10 to about 15,about 15 to about 20.

In various embodiments, the composite layers characterized as having anaverage surface area. While the bounds of these areas are notnecessarily limited to any particular values, in certain preferredembodiments, the average surface or planar area is defined by a range ofareas, with individual embodiments having a lower range value of about50 nm², about 100 nm², about 250 nm², about 500 nm², or about 1000 nm²,and having an upper range value of about 10,000 nm², about 5000 nm²,about 2500 nm², about 1000 nm², about 500 nm², about 250 nm², or about100 nm², with exemplary ranges of about 100 nm² to about 2500 nm², ofabout 250 nm² to about 2500 nm², of about 500 nm² to about 1500 nm², ofabout 500 nm² to about 1000 nm², 50 nm² to about 250 nm², or about 750nm² to about 1000 nm².

In other preferred embodiments, the average surface or planar area isdefined by a range of areas, with individual embodiments having a lowerrange value of about 5 μm², about 10 μm², about 25 μm², about 50 μm²,about 100 μm², about 250 μm², about 500 μm², or about 1000 μm² andhaving an upper range value of about 100,000 μm², 10,000 μm², about 1000μm², about 500 μm², about 250 μm², about 100 μm², about 50 μm², about 25μm², or about 10 μm², with exemplary ranges of about 10 μm² to about 250μm², of about 25 μm² to about 250 μm², of about 50 μm² to about 150 μm²,of about 50 μm² to about 100 μm², 5 μm² to about 25 μm², or about 75 μm²to about 125 μm².

While the surface of these composite layer may be of any shape, it isconvenient to describe such shapes as having a major and minor planardimension (or x-axis and y-axis dimensions, using the envisioned x-yplane as described above). For example, if a quadrilateral orpseudo-quadrilateral shape, the major and minor dimension is the lengthand width dimensions. In preferred embodiments, the ratio of the lengthsof the major and minor axes is in the range of about 1 to about 10(1:10) to about 10 to about 1 (10:1), about 1 to about 5 (1:5) to about5 to about 1 (5:1), more preferably about 1 to about 3 (1:3) to about 3to about 1 (3:1), or about 1 to about 2 (1:2) to about 2 to about 1(2:1).

Additionally, in various embodiments, the interlayer distances (i.e.,the distances between the composite crystal layers) in these stackedassemblies is in the range of about 0.2 nm to about 1 nm, preferably inthe range of about 0.3 nm to about 0.5 nm. When prepared by the methodsdescribed below (i.e., removing the labile A-phase elements from MAXphase materials, see below), these interlayer distances may beconsistent with the atomic radii of the removed elements. For example,the atomic diameter of Al is about 0.25 nm and that of Sn about 0.3 nm.

Certain embodiments of the present invention provide stacked assemblieswhich are capable of intercalating atoms and/or ions between at leastsome of the layers of two-dimensional crystal layers. Such spontaneousintercalation of cations from aqueous solutions was not theoretically orpreviously demonstrated. For example, these atoms and/or ions can bemetal or metalloid atoms or ions, including alkali, alkaline earth, andtransition metals. In some embodiments, these are alkali metal atomsand/or ions (e.g., Li, Na, K, and/or Cs); and most preferably lithium.In other embodiments, the atoms and/or ions include ammonium, magnesium,and aluminum. In some embodiments, these atoms and/or ions are able tomove into and out of the stacked assemblies.

In certain embodiments, the cations intercalated spontaneously, onexposure of the cations to the MX-ene materials, using alkaline oracidic aqueous media (see, e.g., Example 13.2). Carbonates, carboxylates(such as described in Example 13.2), hydroxides, and sulfates may beused to introduce the cations into between the MXene layers. In somecases, notably Al³⁺, the intercalation can additionally be promotedelectrochemically. These intercalated compositions are able to inducehigh capacitances in flexible Ti₃C₂T_(s) paper electrodes in aqueouselectrolytes. Generally, these intercalated structures may beincorporated into electrodes, double layer capacitors, or both, wheresaid structures further comprise, for example, conductive carbon (e.g.,onion-like carbon or carbon black) and fluoropolymer binders (includingperfluorinated binders known in the art, e.g., PTFE).

These multilayer structures or assemblies may be used for the same typesof applications described above for the MXene layer compositions.

Additionally, the ability to intercalate lithium atoms and/or ions,together with the electrical properties of the MXene layers describedabove, provides the opportunities that these stacked assemblies may beused as energy storing devices (e.g., anodes) comprising theseintercalated stacked composition, or the energy storage devicesthemselves, for example, batteries, comprising these elements.

Density functional theory (DFT) calculations at 0 K and in Li-richenvironments show that the formation of Ti₃C₂Li₂ as a result of theintercalation of Li into the space vacated by the Al atoms (FIG. 6( c))assuming reactionTi₃C₂+2Li═Ti₃C₂Li₂  (4)has an enthalpy change of 0.28 eV. One possible reason for the positivevalue maybe the fact that Li has an atomic radius of 145 pm, whereasthat of Al is 125 pm. The structure shown in FIG. 6( c) would provide acapacity of 320 mAhg⁻¹, which is comparable to the 372 mAhg⁻¹ ofgraphite for (LiC₆). More recently, a steady state capacity of ca. 410mAhg⁻¹ has been achieved for MX-ene compositions comprising Ti₃C₂intercalated with Li⁺.

Accordingly, various embodiments of the present invention include Li-ionbatteries (FIG. 6( c)) and pseudo-capacitor electrodes, wherein theMXene layers or assemblies replace layered transition metal oxides,which show useful red-ox properties and Li-intercalation, but which havelower electrical conductivities than described herein for the MXenematerials.

The ability of MXene to intercalate ions, including lithium ions, so asto allow these materials to act as Li-ion batteries and/orpseudo-capacitor electrodes, is shown in Example 10, below. Similarly,the ability to intercalate a wide range of cations from aqueoussolutions (as shown in Example 13), both from multilayer MX-enes andMX-ene “paper” made from a few layers of MX-ene materials, makes theseionically intercalated materials useful for those embodiments comprisingflexible and wearable energy storage devices. The fact that a variety ofions, as different as Na⁺ and Al³⁺, can be accommodated between theMXene layers provide for embodiments comprising batteries as well as inmetal-ion capacitors (battery-supercapacitor hybrids) which comprisethese intercalated MXene as well.

Other embodiments of the present invention provide stacked assemblieswhich are capable of being intercalated or actually are intercalated bysmall molecules or salts thereof between at least some of the layers oftwo-dimensional crystal layers. In this regard, the term “smallmolecules,” describes molecules comprising C, H, N, O, or S, and havingmolecular weights less than about 250 daltons. These molecules or saltsare preferably, but not necessarily, polar. These molecules or salts arepreferably, but not necessarily, aprotic. In some embodiments, thestacked assemblies are capable of being intercalated or actually areintercalated by molecules or salts thereof, said molecules or salt beingthose which are known to intercalate into kaolinite between at leastsome of the layers of two-dimensional crystal layers. In this regard,they may be described to as “kaolinitic intercalators.” Without beingbound to any particular theory, it appears that these intercalatingchemicals are capable of stably interacting with the surfacefunctionalities of the individual layers of the MX-ene materials.Exemplary small molecules or kaolinitic intercalators include hydrazine,hydrazine monohydrate, DMSO, urea, and N, N-dimethylformamide. Ammoniumhydroxide has also been demonstrated to intercalate into these stackedassemblies. N-methylformamide (NMF) and 1-methyl-2-pyrrolidone (NMP) arealso known to intercalate into kaolinite matrices

Example 14 describes some exemplary, non-limiting methods ofintercalating these types of chemicals into the MX-ene matrices. It isnoted that for at least some of these chemicals, the intercalation isreversible—i.e., they can be inserted and removed by varying processingconditions, including simple exposure to the potential intercalant andvariations in temperature, or both. It should also be apparent thatintroducing a first intercalated chemical into a given MX-ene matrix,may provide an opportunity to substitute it by a second chemical orchemicals, perhaps larger organic molecules, either by co- orpost-intercalation, thereby providing a route to a broader class ofintercalated compositions, similar to intercalated kaolinitederivatives. For example, pyrrolidinium halide and benzamideintercalation compounds of kaolinite are known to be available from DMSOintercalated kaolinite, and similar substitutions may be available foranalogous compounds comprising these MX-ene materials. The specificembodiments described in Example 14 are deemed part of the presentinvention.

In addition to the compositions of the MXene materials, variousembodiments provide for the preparation of such materials. Certainembodiments provide methods of preparing compositions comprising: (a)removing substantially all of the A atoms from a MAX-phase compositionhaving an empirical formula of M_(n+1)AX_(n); wherein M is an earlytransition metal or a mixture thereof, wherein A is a so-called A-groupelement (typically described, see below, as including Al, Si, P, S, Ga,Ge, As, Cd, In, Sn, Tl, and Pb); wherein X is C or N, or a combinationthereof; and wherein n=1, 2, or 3 so as to provide a free standingcomposition comprising a framework of a substantially two-dimensionalcomposite crystal layer having first and second surfaces.

MAX phase compositions are generally recognized as comprising layered,hexagonal carbides and nitrides have the general formula: M_(n+1)AX_(n),(MAX) where n=1 to 3, in which M is typically described as an earlytransition metal (comprising a Group IIIB, IVB, VB, or VIB metal, orMn), A is described as an A-group (mostly IIIA and IVA, or groups 13 and14) element and X is either carbon and/or nitrogen. See, e.g., M. W.Barsoum, et al., “Synthesis and Characterization of a RemarkableCeramic: Ti₃SiC₂ ,” J. Amer. Ceramics. Soc., 79, 1953-1956 (1996); M. W.Barsoum, “The M_(N+1)AX_(N) Phases: A New Class of Solids:Thermodynamically Stable Nanolaminates,” Progress in Solid StateChemistry, 28, 201-281 (2000), both of which are incorporated byreference herein. While Ti₃AlC₂ is among the most widely studied ofthese materials, more than 60 MAX phases are currently known to existand are useful in the present invention. While not intending to belimiting, representative examples of MAX phase materials useful in thepresent invention include: (211) Ti₂CdC, Sc₂InC, Ti₂AlC, Ti₂GaC, Ti₂InC,Ti₂TIC, V₂AlC, V₂GaC, Cr₂GaC, Ti₂AlN, Ti₂GaN, Ti₂InN, V₂GaN, Cr₂GaN,Ti₂GeC, Ti₂SnC, Ti₂PbC, V₂GeC, Cr₂AlC, Cr₂GeC, V₂PC, V₂AsC, Ti₂SC,Zr₂InC, Zr₂TlC, Nb₂AlC, Nb₂GaC, Nb₂InC, Mo₂GaC, Zr₂InN, Zr₂TlN, Zr₂SnC,Zr₂PbC, Nb₂SnC, Nb₂PC, Nb₂AsC, Zr₂SC, Nb₂SC, Hf₂InC, Hf₂TlC, Ta₂AlC,Ta₂GaC, Hf₂SnC, Hf₂PbC, Hf₂SnN, Hf₂SC; (312) Ti₃AlC₂, V₃AlC₂, Ti₃SiC₂,Ti₃GeC₂, Ti₃SnC₂, Ta₃AlC₂, and (413) Ti₄AlN₃, V₄AlC₃, Ti₄GaC₃, Ti₄SiC₃,Ti₄GeC₃, Nb₄AlC₃, and Ta₄AlC₃. Solid solutions of these materials canalso be used as described herein (e.g., see Example 4).

MAX phase materials are themselves known to exist as laminatedstructures with anisotropic properties. These materials are layeredhexagonal (space group P6₃/mmc), with two formula units per unit cell(FIG. 1). Near close-packed M-layers are interleaved with pure A-groupelement layers, with the X-atoms filling the octahedral sites betweenthe former.

Within the MAX phase structure, the M_(n+1)X_(n) layers are chemicallyquite stable, possibly owing to the strength of the M-X bond. Bycomparison, the A-group atoms are the most reactive species, reflectiveof their relatively weak binding. For example, heating Ti₃SiC₂ in aC-rich atmosphere or heating in molten cryolite or molten aluminum isknown to result in the loss of Si and the formation of TiC_(x). In thecase of cryolite, the vacancies that form lead to the formation of apartially ordered cubic TiC_(0.67) phase. In both cases, the hightemperatures lead to a structural transformation from a hexagonal to acubic lattice and a partial loss of layering. In some cases, such asTi₂InC, simply heating in vacuum at ca. 800° C., results in loss of theA-group element and TiC_(x) formation. Removing of both the M and Aelements from MAX structure by high temperature chlorination results ina porous carbon known as carbide derived carbon with useful and uniqueproperties.

By contrast, the present methods surprisingly provide for thepreparation of compositions comprising layers or stacked assemblies ofat least one layer having first and second surfaces, each layercomprising a substantially two-dimensional array of crystal cells, eachcrystal cell deriving from the M_(n+1)X_(n) layers of MAX phasecompositions. These compositions are capable of free-standing or can beorganized into stacked assemblies of coated crystal layers.

As used herein, the term “removing substantially all of the A atoms froma MAX-phase composition” connotes embodiments wherein at least 50 atomic% of the A atoms are removed from a finally recovered sample, relativeto the original MAX phase composition. In other more preferredindependent embodiments, more than about 60 atomic %, more than about 70atomic %, more than about 80 atomic %, more than about 90 atomic %, morethan about 95 atomic %, more than about 98 atomic %, and more than about99 atomic % of the A atoms are removed from a finally recovered sample,relative to the original MAX phase composition.

Certain embodiments provide a process for removing these A atomscomprising treatment with an acid, preferably a strong acid capable ofreacting with the A atoms. Such acids may be organic or inorganic acids,and may be applied in the gas or liquid phase, provided the resultingA-atom product can be removed from the lattice. In this regard, strongacids which include fluorine atoms appear to be especially preferred.Aqueous hydrofluoric acid is among those acids which appear especiallyuseful. Aqueous ammonium hydrogen fluoride (NH₄F.HF) is another, moresafely handled, acid which may be useful in effecting removal of the Aatom. Other alkali metal bifluoride salts (i.e., QHF₂, where Q is Li,Na, or K) may also be useful for this purpose. Indeed, even conditionswhich generate aqueous HF in situ (for example, using alkali metalfluoride salts (e.g., NaF) in the presence of mineral acids, such as HClor HNO₃, have been shown to provide mixtures capable of effectivelyremoving the A atom from MAX phase materials. The skilled artisan willalso appreciate that any reactant known to react preferentially with theA atoms of a given MAX phase composition, relative to the M_(n+1)X_(n)may also be useful, for example selective chelants. Uses of suchreactants are considered within the scope of this invention.

The extraction of the A group layers may be done at room, or evenmoderate, temperature, for example in the range of about 20° C. to about800° C., preferably in temperature ranges wherein the lower temperatureis about 20° C., about 25° C., about 30° C., about 40° C., about 50° C.,about 60° C., about 80° C., about 100° C., about 200° C., or about 300°C., and wherein the upper temperature is about 600° C., about 500° C.,about 400° C., about 300° C., about 250° C., about 200° C., about 150°C., about 100° C., about 80° C., or about 60° C. Exemplary examples ofranges include temperatures in the range of about 20° C. to about 100°C., about 20° C. to about 60° C., or about 30° C. to about 60° C. Theextractions may be conducted using liquid or gas phase extractionmethods. Gas phase reactions are generally to be done at the highertemperatures.

In further embodiments, the chemically treated materials are subjectedto sonication, either using ultrasonic or mega sonic energy sources.This sonication may be applied during or after the chemical treatment.

One embodiment of the chemical exfoliation process for onerepresentative material is diagrammatically illustrated in FIG. 3, anddescribed further below. In this example, the treatment of Ti₃AlC₂powders for 2 h in aqueous HF resulted in the formation of exfoliated2-D Ti₃C₂ layers. The term “exfoliated” refers to a process ofdelaminating the individual (or multiple individual layers) from thestacked assemblies (see, e.g., the second step illustrated in FIG. 3).The exposed Ti surfaces appear to be terminated by OH and/or F (seeExamples below). While not intending to be bound by the correctness ofany single theory or mechanism, based on the experimental informationprovided below, it appears that the following simplified reactions occurwhen Ti₃AlC₂ is immersed in aqueous HF:Ti₃AlC₂+3HF═AlF₃+3/2H₂+Ti₃C₂  (1)Ti₃C₂+2H₂O═Ti₃C₂(OH)₂+H₂  (2)Ti₃C₂+2HF═Ti₃C₂F₂+H₂  (3)

Reaction (1) appears to be a necessary step, at least to the extent thatit provides for the extraction of AlF₃ in some form (e.g., perhaps somesoluble derivative, such as H₃AlF₆), followed or accompanied by reaction(2) and/or (3). Evidence consistent with the aforementioned reactionsand that they result in the exfoliation of 2-D Ti₃C₂ layers, with OHand/or F surface groups is presented below. Reactions (2) and (3) aresimplified in that they assume the terminations are OH or F,respectively, when in fact they may be a combination of both.

Non-limiting examples of MXene compositions prepared by chemicalexfoliation are illustrated in FIGS. 9-18.

In other embodiments, the exfoliation can be accomplishedelectrochemically. In various embodiments, MAX phase materials areselectively exfoliated to form the corresponding MXene by theapplication of potentiostatic or galvanostatic polarization. See Example9, below.

It should also be recognized that, in addition to those embodimentsdescribed for the compositions provided above, other embodiments providefor compositions provided by the methods of preparation describedherein. For example, those composition obtained from subjecting a MAXphase material to a chemical exfoliation process, said exfoliationprocess comprising treatment with aqueous HF and sonication, wherein asubstantial portion of the A atoms are removed should also be consideredwithin the scope of the present invention.

The following listing of embodiments is intended to complement, ratherthan displace or supersede, the previous descriptions.

Embodiment 1

A composition comprising at least one layer having first and secondsurfaces, each layer comprising:

a substantially two-dimensional array of crystal cells,

each crystal cell having an empirical formula of M_(n+1)X_(n), such thateach X is positioned within an octahedral array of M,

wherein M is at least one Group IIIB, IVB, VB, or VIB metal, or Mn

wherein each X is C, N, or a combination thereof, and

n=1, 2, or 3.

Embodiment 2

The composition of Embodiment 1 comprising a plurality of layers.

Embodiment 3

The composition of Embodiment 1 or 2, wherein M is at least one GroupIVB, Group VB, or Group VIB metal.

Embodiment 4

The composition of any one of Embodiments 1 to 3, wherein M_(n+1)X_(n)comprises Sc₂C, Sc₂N, Ti₂C, Ti₂N, Mo₂C, V₂C, V₂N, Cr₂C, Cr₂C, Cr₂N, Zr₂C, Zr₂N, Nb₂C, Nb₂N, Hf₂C, Hf₂N, Ti₃C₂, Ti₃N₂, V₃C₂, V₃C₂, Ta₃C₂, Ta₃N₂,or Ti₄C₃, Ti₄N₃, V₄C₃, V₄N₃, Ta₄N₃, or a combination thereof

Embodiment 5

The composition of any one of Embodiments 1 to 4, wherein M_(n+1)X_(n)comprises Ti₃C₂, Ti₃CN, Ti₂C, Ta₄C₃, or (V_(1/2)Cr_(1/2))₃C₂.

Embodiment 6

The composition of any one of Embodiments 1 to 4, wherein M_(n+1)X_(n)comprises V₂C, Mo₂C, Nb₂C₃, and Mo₃C₂.

Embodiment 7

The composition of any one of Embodiments 1 to 5, wherein M is Ti, and nis 1 or 2.

Embodiment 8

The composition of any one of Embodiments 1 to 5, wherein M is Ta.

Embodiment 9

The composition of any one of Embodiments 1 to 7, wherein n=1.

Embodiment 10

The composition of any one of Embodiments 1 to 8, wherein n=2

Embodiment 11

The composition of any one of Embodiments 1 to 5, wherein n=3.

Embodiment 12

The composition of any one of Embodiments 1 to 11, wherein the layer isin the form of a plane, a scroll, or a tube.

Embodiment 13

The composition of any one of Embodiments 1 to 12, wherein at least oneof said surfaces is coated with a coating comprising alkoxide,carboxylate, halide, hydroxide, hydride, oxide, sub-oxide, nitride,sub-nitride, sulfide, thiol, or a combination thereof

Embodiment 14

The composition of Embodiment 13, wherein the coating comprises fluorineatoms or fluoride ions.

Embodiment 15

The composition of Embodiment 13, wherein the coating comprises hydratedor anhydrous oxide or sub-oxide, or combination thereof.

Embodiment 16

The composition of Embodiment 1, the crystal cells having an empiricalformula Ti₃C₂ or Ti₂C and wherein at least one of said surfaces iscoated with a coating comprising hydroxide, oxide, sub-oxide, or acombination thereof

Embodiment 17

The composition of Embodiment 1, the crystal cells having an empiricalformula Ta₄C₃ and wherein at least one of said surfaces is coated with acoating comprising hydroxide, oxide, sub-oxide, or a combination thereof

Embodiment 18

The composition of any one of Embodiments 1 to 17, wherein thecomposition comprises an electrically conductive or semiconductivesurface.

Embodiment 19

A polymer composite comprising an organic polymer and the composition ofany one of Embodiments 1 to 18.

Embodiment 20

The copolymer composite of Embodiment 19, wherein the polymer compositeis in a form having a configuration defined by a two-dimensional plane,wherein the organic polymer is oriented coincident with the plane ofthat planar configuration.

Embodiment 21

The polymer composite of Embodiment 19 or 20, wherein the substantiallytwo-dimensional array of crystal cells defines a plane, and said planeis substantially aligned with the plane of the polymer composite.

Embodiment 22

The polymer composite of any one of Embodiments 19 to 21, wherein theflexural strength and/or stiffness in the planar dimension of thepolymer composite is greater than the flexural strength and/or stiffnessof a corresponding polymer composition without the composition of claim1.

Embodiment 23

A stacked assembly of at least two layers having first and secondsurfaces, each layer comprising:

a substantially two-dimensional array of crystal cells,

each crystal cell having the empirical formula of M_(n+1)X_(n), suchthat each X is positioned within an octahedral array of M;

wherein M is a Group IIIB, IVB, VB, or VIB metal, or Mn;

each X is C, N, or a combination thereof; and

n=1, 2, or 3;

wherein the layers are characterized as having an average surface areaand interlayer distance.

Embodiment 24

The stacked assembly of Embodiment 23, wherein M_(n+1)X_(n) comprisesSc₂C, Sc₂N, Ti₂C, Ti₂N, Mo₂C, V₂C, V₂N, Cr₂C, Cr₂N, Zr₂C, Zr₂N, Nb₂C,Nb₂N, Hf₂C, Hf₂N, Ti₃C₂, Ti₃N₂, V₃C₂, V₃C₂, Ta₃C₂, Ta₃N₂, or Ti₄C₃,Ti₄N₃, V₄C₃, V₄N₃, Ta₄C₃Ta₄N₃ or a combination thereof.

Embodiment 25

The stacked assembly of Embodiment 23, wherein M_(n+1)X_(n) comprisesTi₂C, Ti₂N, Ti₃C₂, Ti₃N₂, Ti₄C₃, Ti₄N₃, Ta₃C₂, Ta₃N₂, Ta₄C₃, or Ta₄N₃,or a combination thereof.

Embodiment 26

The stacked assembly of any one of Embodiments 23 to 25, whereinM_(n+1)X_(n) comprises Ti₃C₂, TiNbC, Nb₂C, Ti₃CN, Ti₂C, Ta₄C₃, or(V_(1/2)Cr_(1/2))₃C₂.

Embodiment 27

The stacked assembly of Embodiments 23 or 24, wherein M_(n+1)X_(n)comprises V₂C, Mo₂C, Nb₂C₃, and Mo₃C₂.

Embodiment 28

The stacked assembly of Embodiments 23, wherein M_(n+1)X_(n) is Ti₃C₂,TiNbC, Ti₃CN, or Ti₂C.

Embodiment 29

The stacked assembly of Embodiments 23, wherein M_(n+1)X_(n) is Ti₃C₂.

Embodiment 30

The stacked assembly of any one of Embodiments 23 to 29, wherein atleast one of the surfaces has bound thereto alkoxide, carboxylate,halide, hydroxide, hydride, oxide, sub-oxide, nitride, sub-nitride,sulfide, thiol, or a combination thereof

Embodiment 31

The stacked assembly of any one of Embodiments 23 to 30, wherein thenumber of layers is in the range of 2 to about 50.

Embodiment 32

The stacked assembly of any one of Embodiments 23 to 29, wherein theaverage area of the layers is in the range of about 100 nm² to about10,000 nm² or about 100 μm² to about 10,000 μm².

Embodiment 33

The composition of any one of Embodiments 23 to 30, capable ofintercalating atoms and/or ions between at least some of the layers.

Embodiment 34

The stacked assembly of any one of Embodiments 23 to 31, said assemblyresulting from the deposition of delaminated flakes of the compositiononto a surface.

Embodiment 35

The stacked assembly of any one of Embodiments 23 to 34, said assemblyexhibiting at least 65% transparency to at least one wavelength of lightin a range of from about 250 nm to about 850 nm and a surfaceresistivity of less than about 50 micro-ohm-meter.

Embodiment 36

The stacked assembly of any one of Embodiments 23 to 35, wherein atoms,ions, or both atoms and ions of the same material are intercalatedbetween at least some of the layers.

Embodiment 37

The stacked assembly of any one of Embodiments 23 to 36, wherein theatoms, ions, or both atoms and ions of the same material compriselithium, sodium, potassium, magnesium, or a combination thereof.

Embodiment 38

The stacked assembly of Embodiment 37, wherein the atoms, ions, or bothatoms and ions of the same material comprise or consist essentially oflithium.

Embodiment 39

The stacked assembly of any one of Embodiments 23 to 38, furthercomprising a kaolinitic intercalator intercalated between at least someof the layers.

Embodiment 40

The stacked assembly of Embodiment 39, wherein the kaoliniticintercalator is hydrazine, DMSO, urea or N, N-dimethylformamide.

Embodiment 41

An energy-storing device or electrode comprising the stacked assembly ofany one of Embodiments 23 to 40.

Embodiment 41

A battery comprising the stacked assembly of any one of Embodiments 23to 40.

Embodiment 42

A method of preparing a composition comprising: removing substantiallyall of the A atoms from a MAX-phase composition having an empiricalformula of M_(n+1)AX_(n).

wherein M is at least one Group IIIB, IVB, VB, or VIB metal, or Mn,

wherein A is an A-group element;

each X is C, N, or a combination thereof; and

n=1, 2, or 3,

thereby providing a composition comprising at least one layer having afirst and second surface, each layer comprising a substantiallytwo-dimensional array of crystal cells.

Embodiment 43

The method of Embodiment 42, wherein the A atoms are removed by aprocess comprising a treatment with a fluorine-containing acid.

Embodiment 44

The method of Embodiments 43, wherein the fluorine-containing acid isaqueous hydrofluoric acid.

Embodiment 45

The method of Embodiment 43, wherein the fluorine-containing acid is asubstantially anhydrous gas.

Embodiment 46

The method of Embodiment 43, wherein the fluorine-containing acidcomprises aqueous ammonium hydrogen fluoride (NH₄F.HF), NaHF₂, or amixture resulting from the combination of an alkali metal salt with amineral acid.

Embodiment 47

The method of any one of Embodiments 42 to 46, further comprisingsonication.

EXAMPLES

The following Examples are provided to illustrate some of the conceptsdescribed within this disclosure. While each Example is considered toprovide specific individual embodiments of composition, methods ofpreparation and use, none of the Examples should be considered to limitthe more general embodiments described herein.

In the following examples, efforts have been made to ensure accuracywith respect to numbers used (e.g. amounts, temperature, etc.) but someexperimental error and deviation should be accounted for. Unlessindicated otherwise, temperature is in degrees C., pressure is at ornear atmospheric.

Example 1 Methods and Materials

Powder of Ti₃AlC₂ was prepared by ball-milling Ti₂AlC (>92 wt. %3-ONE-2, Voorhees, N.J.) and TiC (99% Johnson Matthey Electronic, N.Y.)powders in a 1:1 molar ratio for 24 h using zirconia balls. The mixturewas heated to 1350° C. for 2 h under argon, Ar. The resulting looselyheld compact was crushed in a mortar and pestle. Roughly 10 g of powdersare then immersed in ≈100 ml of a 50% concentrated hydrofluoric acid,HF, (Fisher Scientific, Fair Lawn, N.J.) solution at room temperaturefor 2 h. The resulting suspension was then washed several times usingde-ionized water and centrifuged to separate the powders. In some cases,to align the flakes and produce free-standing discs, the treated powderswere cold pressed at a load corresponding to a stress of about 1 GPa ina steel die.

X-ray diffraction (XRD) patterns were obtained with a powderdiffractometer (Siemens D500, Germany) using Cu K_(α) radiation, and astep scan of 0.02° and 1 s per step. Si powder was added to some samplesas an internal standard. A scanning electron microscope, (SEM, ZeissSupra 50VP, Germany) was used to obtain high magnification images of thetreated powders. Transmission electron microscopes, TEMs, (JEOLJEM-2100F and JEM 2100, Japan; FEI, Tecnai G2 TF20UT FEG, Netherlands)operating at 200 kV were used to characterize the exfoliated powders.Chemical analysis in the TEM was carried out using an ultra-thin windowX-ray energy dispersive spectrometer, EDAX (EDAX, Mahwah, N.J.). The TEMsamples were prepared by deposition of the flakes—from an isopropanolsuspension—on a lacey-200 mesh carbon-coated copper grid. Ramanspectroscopy of the cold pressed samples was carried out on amicrospectrometer (inVia, Renishaw plc, Gloucestershire, UK) using an Arion laser (514.5 nm) and a grating with 1800 lines/mm. This correspondsto a spectral resolution of 1.9 cm⁻¹ and a spot size of 0.7 μm in thefocal plane. X-ray photoelectron spectroscopy, XPS, (PHI 5000,ULVAC-PHI, Inc., Japan) was used to analyze the surfaces of samplesbefore and after exfoliation.

Theoretical calculations were performed by density functional theory(DFT) using the plane-wave pseudo-potential approach, with ultrasoftpseudopotentials and Perdew Burke Ernzerhof (PBE) exchange—Wu-Cohen (WC)correlation functional, as implemented in the CASTEP code in MaterialStudio software (Version 4.5). A 8×8×1 Monkhorst-Pack grid and planewavebasis set cutoff of 500 eV were used for the calculations. Exfoliationwas modeled by first removing Al atoms from the Ti₃AlC₂ lattice. ExposedTi atoms located on the bottom and top of the remaining Ti₃C₂ layerswere saturated by OH (FIG. 3B) or F groups followed by full geometryoptimization until all components of the residual forces became lessthan 0.01 eV/Å. Equilibrium structures for exfoliated layers weredetermined by separating single Ti₃C₂ layers by a 1.2 nm thick vacuumspace in a periodic supercell followed by the aforementioned fullgeometry optimization. Band structures of the optimized materials werecalculated using a k point separation of 0.015 Å⁻¹. The elasticproperties of the 2-D structures were calculated by subjecting theoptimized structure to various strains and calculating the resultingsecond derivatives of the energy density.

Example 2 Experimental Characterization of Ti₃C₂(OH)₂ and Ti₃C₂(F)₂

XRD spectra of the initial Ti₂AlC—TiC mixture after heating to 1350° C.for 2 h resulted in peaks that corresponded mainly to Ti₃AlC₂ (bottomcurve in FIG. 4( a)). When the Ti₃AlC₂ powders were placed into the HFsolution, bubbles, believed to be H₂, were observed suggesting achemical reaction. Ultrasonication of the reaction products in methanolfor 300 s resulted in significant weakening of the peaks and theappearance of an amorphous broad band around 24° (top spectrum in FIG.4( a)). In other words, exfoliation leads to a loss of diffractionsignal in the out-of-plane direction, and the non-planar shape of thenanosheets results in broadening of peaks corresponding to in-planediffraction. When the same powders were cold pressed at 1 GPa, intofree-standing, 300 μm thick and 25 mm diameter discs (FIG. 4( e)), theirXRD showed that most of the non-basal plane peaks of Ti₃AlC₂—mostnotably the most intense peak at ≈39°—disappear (curve labeled “HFetched” in FIG. 4( a)). On the other hand, the (001) peaks, such as the(002), (004) and (010), broadened, lost intensity, and shifted to lowerangles compared to their location before treatment. Using the Scherrerformula, as described in B. D. Cullity, Elements of X-ray diffraction,Addison-Wesley 1978, which is incorporated by reference herein, theaverage particle dimension in the [0001] direction after treatment isestimated to be 11±3 nm, which corresponds to roughly ten Ti₃C₂(OH)₂layers. To identify the peaks we simulated XRD patterns of hydroxylated,viz. Ti₃C₂(OH)₂, (curve labeled “Ti₃C₂(OH)₂” in FIG. 4( a)) andfluorinated, Ti₃C₂F₂, structures (curved labeled as such in FIG. 4( a)).Clearly, both were in good agreement with the XRD patterns of thepressed sample (curve labeled “HF etched” in FIG. 4( a)), the agreementwas better with the former. The disappearance of the most intensediffraction peak of Ti₃AlC₂ at 39° and the good agreement between thesimulated XRD spectra for Ti₃C₂(OH)₂ and the experimental resultsprovides strong evidence of the formation of the latter. The presence ofOH groups after treatment was confirmed by FTIR.

Further DFT geometry optimization of the hydroxylated (FIG. 5( f)) andfluorinated structure resulted in 5% and 16% expansion of the originalTi₃AlC₂ lattice, respectively, as observed. If Al were simply removed,and not replaced by functional groups, the DFT optimization caused thestructure to contract by 19%, which is not observed. The increase of thec-lattice parameters upon reaction (FIG. 4( a)) is thus strong evidencefor the validity of reactions 2, 3.

Raman spectra of Ti₃AlC₂, before and after HF treatment, are shown inFIG. 4( b). Peaks II, III, and IV vanished after treatment, while peaksVI and VII, merged, broadened and downshifted. Such downshifting hasbeen observed in Raman spectra of very thin layers of inorganic layeredcompounds, and is characteristic of such materials. See, e.g., C. N. R.Rao, et al., Science and Technology of Advanced Materials 2010, 11,054502, which is incorporated by reference herein in its entirety. Theline broadening, and the spectral shifts in the Raman spectra areconsistent with exfoliation and are in agreement with the broadened XRDprofiles. In analogy with Ti₃SiC₂ (see J. Spanier, S. Gupta, M. Amer, M.W. Barsoum, Physical Review B 2005, 71, 012103, which is alsoincorporated by reference herein), peaks I to III in FIG. 4( b) can beassigned to Al—Ti vibrations, while peaks V and VI involve only Ti—Cvibrations. The fact that only the latter two exist after etchingconfirms both the mode assignments, but more importantly the loss of Alfrom the structure. Note that peaks V and VI are combined, broadened anddownshifted after reaction.

The Ti 2p XPS spectra, before and after treatment, are shown in FIG. 4(c). The C is and Ti 2p peaks before treatment match previous work onTi₃AlC₂. See, e.g., S. Myhra, et al., Journal of Physics and Chemistryof Solids 2001, 62, 811, which is incorporated by reference herein. Thepresence of Ti—C and Ti—O bonds was evident from both spectra,indicating the formation of Ti₃C₂(OH)₂ after treatment. The Al and Fpeaks (not shown) were also observed and their concentrations werecalculated to be around 3 at.% and 12 at.%, respectively. Aluminumfluoride (AlF₃)—a reaction product, see below—can probably account formost of the F signal seen in the spectra. The O 1s main signal (notshown at ˜530.3 cm⁻¹) suggest the presence of OH group. See, e.g., M.Schmidt, S. G. Steinemann, Fresenius' Journal of Analytical Chemistry1991, 341, 412, which is also incorporated by reference herein.

A SEM image of a ≈1500 μm³ Ti₃AlC₂ particle (FIG. 4( d)) shows how thebasal planes fan out and spread apart as a result of the HF treatment.EDAX of the particles showed them to be comprised of Ti, C, O and F,with little, or no, Al. This implies that the Al layers were replaced byoxygen (i.e. OH) and/or F. Note that the exfoliated particles maintainedthe pseudo-ductility of Ti₃AlC₂ and could be easily cold press intofreestanding disks (FIG. 4( e)). This property can prove crucial in somepotential applications, such as anodes for Li-ion batteries, asdescribed above.

TEM analysis of exfoliated sheets (FIG. 5( a), (b)) shows them to bequite thin and transparent to electrons since the carbon grid is clearlyseen below them. This fact strongly suggests a very thin foil,especially considering the high atomic number of Ti. The correspondingselected area diffraction, SAD (inset in FIG. 5( b)) shows the hexagonalsymmetry of the basal planes. EDAX of the same flake showed the presenceof Ti, C, O, and F. FIG. 5( c), (d) show cross-sections of exfoliatedsingle- and double-layer MXene sheets. FIG. 5( e), (f) showhigh-resolution TEM micrographs and a simulated structure of twoadjacent OH-terminated Ti₃C₂ sheets, respectively. The experimentallyobserved interplanar distances and angles are found to be in goodagreement with the calculated structure. FIG. 6( a), (b) show stackedmultilayer MXene sheets. The exfoliated layers can apparently also berolled into conical shapes (FIG. 6( d)); some are bent to radii of <20nm (FIG. 6( e)). Note that if Al atoms had been replaced by C atoms, theconcomitant formation of strong Ti—C bonds—as when, for example, Ti₃SiC₂reacts with cryolite at 900° C.—exfoliation would not have beenpossible. It follows that the reaction must have resulted in a solid inwhich the Ti—Al bonds are replaced by much weaker hydrogen or van derWaals bonds. This comment notwithstanding, the EDAX results consistentlyshow the presence of F in the reaction products implying that, as notedabove, the terminations are most likely a mixture of F and OH. Thepresence of up to 12 at.% F has also been confirmed using XPS. In thelatter case, however, some of it could originate from AlF₃ residue inthe sample.

Lastly, it is instructive to point out the similarities between MXeneand graphene such as,

-   -   (i) the exfoliation of 2-D Ti₃C₂ layers (FIGS. 6( a) and (b))        into multilayer sheets that resemble exfoliated graphite,        see L. M. Viculis, et al., Journal of Materials Chemistry 2005,        15, 974, which is incorporated by reference herein.    -   (ii) the formation of scrolls (FIGS. 6( d) and (e)).

Also, as cross-sectional TEM (FIG. 6( e)) shows, some nanosheets werebent to radii <20 nm without fracture, which is evidence for strong andflexible Ti₃C₂ layers. Similar scrolls were produced by sonication ofgraphene. See, e.g., L. M. Viculis, et al., Science 2003, 299, 1361; M.V. Savoskin, et al., Carbon 2007, 45, 2797, both of which areincorporated by reference herein. It is possible that the sonicationused for exfoliation caused some nanosheets to roll into scrolls, asschematically shown in FIG. 6( f).

Example 3 Experimental Characterization of the Product of the ReactionBetween Ta₄AlC₃ and Aqueous HF—Ta₄C₃(OH)_(x)(F)_(y)

Ta₄AlC₃ powder (ca. 10 g) was immersed in approximately 100 mL of a 50%concentrated hydrofluoric acid, HF, (Fisher Scientific, Fair Lawn, N.J.)solution at room temperature for 72 h. The resulting suspension was thenwashed several times using deionized water and centrifuged to separatethe powders.

XRD analysis of the resulting material showed sharp peaks correspondingonly to TaC, known to be an impurity in the starting material (i.e., inaddition to peaks attributable to TaC, the XRD spectrum contained onlybroad peaks centered around 2θ values of ca. 6° and 34-36°. However, theXRD spectrum of a sample obtained by cold pressing the resultingmaterial, showed strong, albeit broadened peaks at about 2θ=5.7° and6.8° (apparently shifted from 2θ˜7.5 in XRD of Ta₄AlC₃), smaller peaksat about 2θ=13° (apparently shifted from 20˜15° in XRD of Ta₄AlC₃), 26°,and 29°, and broad, albeit low intensity peaks centered at about2θ=27-30° and 36°, none of which appear to correspond to TaC, but whichare interpreted as being consistent with simulated spectra ofTa₄C₃(OH)₂. Compared with the XRD spectra of the original XRD spectrumof Ta₄AlC₃ (and its an accompanying pattern simulated by CrystalMaker®),the XRD pattern of the cold-pressed material also showed no evidence ofotherwise distinguishing peaks at 2θ˜22°, 29.5°.

An illustrative XRD spectrum for an exfoliated, characterized to beTa₄C₃(OH)_(x)(F)_(y), are shown in FIG. 10.

Example 4 Experimental Characterization of the Product of the ReactionBetween Ti₂AlC and Aqueous HF—Ti₂C(OH)_(x)(F)_(y)

Ti₂AlC powder (Kanthal Corp., Sweden) was immersed in approximately 100mL of a 10% concentrated hydrofluoric acid, HF, (Fisher Scientific, FairLawn, N.J.) solution at room temperature for 10 h. The resultingsuspension was then washed several times using deionized water andcentrifuged to separate the powders. SEM micrographs and XRD spectra ofthe resulting materials are shown in FIG. 8.

Example 5 Experimental Characterization of the Product of the ReactionBetween TiNbAlC and Aqueous HF)—TiNbC (OH)_(x)(F)_(y)

The TiNbAlC powders were made by mixing elemental titanium, Ti (AlfaAesar, Ward Hill, USA, 99.5 wt % purity; 325 mesh), niobium, Nb(Atlantic Equipment Engineers, Bergenfield, USA, 99.8 wt % purity; 325mesh), and the same Al and C used above, in the molar ratio of1:1:1.2:1, respectively, in a ball mill for 12 h. The powders were thenheated at the rate of 10° C./min in a tube furnace to 1500° C. for 1 hunder flowing Ar. After cooling to room temperature, powders wereprocessed as described above (see Table 1). SEM micrographs and XRDspectra of the resulting materials are shown in FIG. 9.

The XRD patterns for TiNbAlC, before and after HF treatment (FIG. 9),show that the intensity of the TiNbAlC peaks decreased significantlyafter HF treatment (considering that 10 wt % Si was used as an internalreference) and a new broad peak at ≈11.8° 2θ appeared after coldpressing. Here again a shoulder at a larger d spacing compared to themain peak is observed. The latter is most likely due to some exfoliated(Ti_(0.5),NB_(0.5))₃AlC₂ that was present as a second phase in thestarting powder. SEM micrographs (FIG. 9) clearly show exfoliatedTiNbAlC particles. TEM micrographs, after sonication (not shown), showthin sheets composed of Ti, Nb, C, O, and F in an atomic ratio that EDXshows to be 14:16:23:34:13, respectively. HRTEM of a TiNbC layer (notshown) and its corresponding SAED again show hexagonal symmetry. At0.2606 nm, the perpendicular separation of the (1010) lattice planesresults in an a lattice constant of 0.301 nm. EELS for TiNbAlC after HFtreatment and confirms the presence of Ti, Nb, C, F (not shown), and O,but no Al.

Example 6 Experimental Characterization of the Product of the ReactionBetween (V_(1/2)Cr_(1/2))₃AlC₂ and AqueousHF)—(V_(1/2)Cr_(1/2))₃C₂(OH)_(x)(F)_(y)

(V_(1/2)Cr_(1/2))₃AlC₂ powder was made by ball milling powders of1.5V+1.5Cr+1.2Al+2C (molar ratios) for 12 hours, then heating themixture under Ar to 1550° C., soaking at this temperature for 2 hours,and cooling to room temperature, after which a powder was obtained fromthe sintered mass using diamond coated milling bit. The powders werethen exfoliated by stirring them in 50% aqueous HF at room temperaturefor 65 hr (5 gm powder in 50 mL acid). SEM micrographs and XRD spectraof the resulting materials are shown in FIGS. 14-16.

Example 7 Experimental Characterization of the Product of the ReactionBetween Ti₃Al(CN) and Aqueous HF—Ti₃(CN)(OH)_(a)(F)_(b)

Ti₃Al(CN) powder was prepared was made by ball milling Ti:AlN:C=3:1:1(molar ratios) for 12 hours, then heating the mixture at 10° C./min to1500° C., holding 2 hours, then cooling, all under Argon (C and Tipowders were purchased from Alfa Aesar, Ward Hill, Mass.). AN powder waspurchased from Sigma-Aldrich. The resulting material was crushed usingmortar and pestle. The resulting powder was immersed and stirred in 30%concentrated hydrofluoric acid, HF, (Fisher Scientific, Fair Lawn, N.J.)solution at room temperature for 18 h. The resulting suspension was thenwashed several times using deionized water and centrifuged to separatethe powders. SEM micrographs and XRD spectra of the resulting materialsare shown in FIGS. 11-13.

Example 8 Effect of Chemical Exfoliation Processing Conditions onFormation and Yield of MXene Compositions

Starting with Ti₃AlC₂ powders as a representative material, a series ofexperiments were conducted to determine the effects of various processparameters on the chemical exfoliation of MAX phase materials to formthe corresponding MXene compositions. In evaluating the effect oftemperature on exfoliation, Ti₃AlC₂ powders were stirred in 50% aqueousHF for 2 hours at different temperatures (e.g., 20, 30, 40, 50, and 65°C.). The effect of processing time was studied by stirring Ti₃AlC₂powders with 50% aqueous HF for 2 hours at room temperature over thetime range of 2 to 19 hours. In testing the effect of initial particlesize, Ti₃AlC₂ powders were crushed in ball milling machine and separatedwith sieves first, then exfoliated by stirring with 50% aqueous HF atroom temperature for 2 hours. FIGS. 19, 20, 21, and 22 illustrate theeffect of HF temperature, time of treatment, and initial particle size,respectively. The specific conditions employed, where different thanthose described above, are provided in each figure.

TABLE 1 List of MAX Phases Exfoliated in This Work and ExfoliationProcess Parameters^(a) c lattice constant HF (nm) Domain Conc TimeBefore After size Yield Compound (%) (hr) HF HF (nm) (wt %) Ti₂AlC 10 101.36 4.504 6 60 Ta₄AlC₃ 50 72 2.408 3.034 38 90 2.843 18 TiNbAlC 50 281.379 1.488 5 80 (V_(0.5)Cr0.₅)₃AlC₂ 50 69 1.773 2.426 28 NA Ti₃AlCN 3018 1.841 2.228 7 80 Ti₃AlC₂ 30 2 1.842 2.051 11 100 Nb₂AlC 50 90 1.3882.234 5 100 Nb₄AlC₃ 50 90 2.419 3.047 27 100 V₂AlC 50 90 1.313 1.973 1060 Mo₂GaC 50 96 1.317 2.021 15 NA ^(a)The particle size for all MAXphases was <35 μm prior to exfoliation. The effects of HF treatment onthe c lattice constant and the average domain size along [0001] deducedfrom the FWHM and the Scherrer formula are listed. The penultimatecolumn shows the estimated process yields.

Example 9 Preparation of MXene Compositions by the ElectrochemicalExfoliation of MAX Phase Materials

Ti₂SnC was made by ball milling 2Ti+Sn+C (molar ratios) for 12 hr, thenheating the mixture at a ramp rate of 10° C./minute to 1250° C., holdingfor 2 hours and cooling to room temperature, all under Ar atmosphere.The resulting material was crushed using mortar and pestle to form apowder (Ti, Sn, and C powders were purchased from Alfa Aesar, Ward Hill,Mass.). Exfoliation of Ti₂SnC was demonstrated by selectivelyelectrochemically removing Sn upon application of a repeated sequencecomposed of a short cathodic polarization (either potentiostatic orgalvanostatic) followed by a long anodic polarization (eitherpotentiostatic or galvanostatic) to an electrochemical system (see FIG.25 for a representative set of conditions; SWPP=square wave potentialpolarization; SWCP=square wave current polarization. Δm_(tot) refers tothe loss in sample weight as a result of the electrochemical treatment).In this system, a hot pressed sample of Ti₂SnC was used as the anode,and Pt was used as the reference and working electrode. The electrolytewas either aqueous 5 M or 12 M HCl, and high purity Ar gas wasconstantly purged through the working solution to maintain an inertatmosphere.

The rapid electrochemical corrosion of the anode material resulted inthe formation of a finely dispersed powder which was collected at thebottom of the reaction vessel, washed with deionized water, and dried.The dried powder was subjected to a series of tests, the results ofwhich are shown in FIGS. 25-28. FIG. 26 shows the dramatic difference inRaman spectra between the product (curves a and b) and the startingmaterial (curves c-e), consistent with the changes seen in other similartransformations (compare, for example, the curves in FIG. 4( b)).Similarly, changes in the XRD spectra (FIG. 27) are indicative of theabsence of starting material. Finally, EDX spectra shown in FIG. 28(A-C)show that the powder is devoid of appreciable Sn, confirming itselimination (Note: the presence of O in these EDX spectra is consistentwith a surface coating of the MXene comprising oxide or hydroxide. Thepresence of Si in the spectra is attributed to the substrate used in themeasurement.

Example 10 Intercalation of Lithium and Use of Ti₃C₂ in Batteries

The electrochemical behavior of MXene compositions (exfoliated MX phasecompositions) was compared to the corresponding MAX phase material inlithium ion battery tests. [The electrolyte used was a mixture ofethylene carbonate and dimethyl carbonate (EC/DMC) with lithiumhexafluorophosphate (LiPF₆). After cell assembly inside a glove box,both Galvanostatic (GV) and Cyclic Voltammetry (CV) tests were used tostudy the electrochemical behavior of MAX phases in Li batteries. Theseelectrochemical tests were carried out using a BioLogic VMP-4potentiostat/galvanostat.] Electrodes were prepared using MAX phase andMXene compositions in a number of electrode configurations, including(a) cold pressed electrode with neither binder nor carbon black; (b)film of powder on copper foil with binder and without carbon black; (c)film f powder on copper foil with binder and carbon black; and (d) filmof carbon black alone with a polyvinylidene-difluoride, PVDF, binder. CVand GV techniques were used to characterize the electrochemical natureof the resulting electrodes/cells. FIG. 29(A/B) shows the results wherethe performance of electrodes prepared using carbon black (CB) andbinder, comparing the additional presence of Ti₃AlC₂ and exfoliatedTi₃AlC₂; i.e., MXene Ti₃C₂(OH)_(x)(F)_(y). As shown in FIG. 29, thecapacity of the MXene containing compositions showed significantlyhigher capacity an order of magnitude higher) than a comparableelectrode made from the corresponding MAX phase material. It is knownthat lithium capacity in MAX phase materials is extremely low, owing tothe lack of space between the layers into which ions may migrate. Thesignificant increase in capacity with the electrodes containing theMXene composition results are consistent with themigration/intercalation of lithium within the stacked layers of MXene

Example 11 Intercalation of Lithium and Use of Ti₂C in Batteries

Testing comparable to that described in Example 10 with Ti₃C₂ (derivedfrom Ti₃AlC₂) was also done with Ti₂C derived from Ti₂AlC. As describedbelow, testing demonstrated the insertion of Li into a newtwo-dimensional (2-D) layered Ti₂C-based material (MXene) with anoxidized surface, formed by etching Al from Ti₂AlC in HF at roomtemperature. Nitrogen sorption of treated powders showed desorptionhysteresis consistent with the presence of slit-like pores. At 23m²·g⁻¹, the specific surface area was an order of magnitude higher thanuntreated Ti₂AlC. Cyclic voltammetry exhibited lithiation anddelithiation peaks at 1.6 V and 2 V vs. Li⁺/Li, respectively. At C/25,the steady state capacity was 225 mAh·g⁻¹; at 1 C, it was 110 mAh·g⁻¹after 80 cycles; at 3 C, it was 80 mAh·g⁻¹ after 120 cycles; at 10 C, itwas 70 mAh·g⁻¹ after 200 cycles.

Pre-reacted, −325 mesh, Ti₂AlC powders were commercially obtained(3-ONE-2, Voorhees, N.J., >92 wt. % purity). The exfoliation process wascarried by immersing the Ti₂AlC powder in diluted (10%) hydrofluoricacid, HF, (Fisher Scientific, Fair Lawn, N.J.) for 10 h at roomtemperature, as described above. The materials were characterized by SEM(Zeiss Supra 50VP, Germany), EDS (Oxford Inca X-Sight, Oxfordshire, UK),and gas sorption analysis (Quantachrome Autosorb-1 with N₂ adsorbate) asdescribed above (i.e., samples were outgassed under vacuum at 200° C.for 48 h. Nitrogen sorption analysis at 77 K was used for calculatingthe specific surface area (SSA) using the Brunauer-Emmet-Teller (BET)equation).

X-ray diffraction, XRD, of the reacted powders indicated that the Al wasselectively etched from the structure. EDS confirmed that the Al layerswere replaced by O and F. SEM images of Ti₂AlC particles after HFtreatment (FIG. 30A) resemble images of exfoliated graphite and clearlyshow HF-induced delamination that are typical of MXenes.

The N₂ sorption isotherm of the treated powders (FIG. 30B) has ahysteresis loop with indications of the presence of mesopores and ashape typical for slit pores. The SSA calculated using the BET equation,for the HF treated Ti₂AlC was 23 m²·g⁻¹. This value is about an order ofmagnitude times higher than the as-received Ti₂AlC powders measured at≈2.5 m²·g⁻¹.

The electrochemical behavior of exfoliated Ti₂AlC in Li batteries wasinvestigated using coin cells (CR 2016) prepared as follows. The workingelectrodes were made with 80 wt % Ti₂C (as described above) and 10 wt. %Super P carbon black mixed with 10 wt. % Poly(vinylidene fluoride)dissolved in 1-Methyl-2-pyrrolidinone. The mixture was then spread ontoa copper foil and dried at ca. 200° C. for 12 h, under a mechanicalvacuum. CR 2016 coin-type cells were assembled using MXene as thepositive electrode and Li metal foil as the negative electrode,separated by a sheet of borosilicate glass fiber (Whatman GF/A)separator saturated with 1 M LiPF₆ solution in a 1:1 weight mixture ofethylene carbonate and diethyl carbonate (EC:DEC) as the electrolyte.The cells were assembled inside an Ar-filled glove box with H₂O and O₂contents <1 ppm, to avoid any moisture contamination.

The cells were subjected to cyclic voltammetry and galvanostaticcharge-discharge cycling using a potentiostat (VMP4, Biologic, S.A.).Electrochemical characterization was typically performed between 0.05 Vand 2.5 V vs. Li⁺/Li.

Typical cyclic voltammetry curves, at a rate of 0.2 mV·s⁻¹, for theexfoliated Ti₂C are shown in FIG. 31A. A broad, irreversible peak wasobserved around 0.6 V, during the first lithiation cycle (reduction); itwas absent in subsequent cycles. This irreversible peak was assigned tothe formation of a solid electrolyte interphase (SEI) and to anirreversible reaction with the electrode material. In all subsequentcycles, broad reversible peaks were observed at 1.6 V and 2.0 V vs.Li⁺/Li during lithiation and de-lithiation, respectively. Because thesepeak potentials are similar to those reported for TiO₂ and lithiatedtitania, these peaks were tentatively assigned to the following redoxreaction:Ti₂CO_(x) +yLi⁺ +ye ⁻

Li_(y)Ti₂CO_(x)  (1)

The rationale for this assignment is that drying at 200° C., prior toassembling the coin cells, rids MXene of water or any OH species andleads to an oxygen terminated surface. In other words, the assumption ismade that the Ti₂CO_(x) surface is similar to that of titania. Like inthe case of the titanates, even if the potentials vs. Li are relativelyhigh, it is an advantage from a safety stand point. Ex situ XRD results(not shown) after lithiation produced no new peaks, but a downshift ofthe MXene peaks was observed, with an increase of the c parameter by19.5% which indicates intercalation of Li between the MXene layers, andnot a conversion reaction.

FIG. 31(B) shows the galvanostatic charge/discharge curves at a rate ofC/10 (1 Li⁺ per formulae exchanged in 10 h). The capacity loss in thefirst cycle can again be attributed to a SEI layer formation atpotentials below 0.9V vs. Li⁺/Li, as well as to the irreversiblereduction of electrochemically active surface groups such as fluorine orpossibly hydroxyls. The specific capacity stabilized after five cyclesat ≈160 mAh·g⁻¹. This value corresponds to y≈0.75 in reaction 1.

At 160 mAh·g⁻¹, the capacity of the treated powders is about 5 timeshigher than that of the as-received Ti₂AlC (ca. 30 mAh·g⁻¹ at C/10)powders. This increase in capacity is traceable to the higher surfacearea, more open structure and weaker bonds between the MX layers afterHF treatment. In addition to the morphological changes, the Li insertionsites are also now different (i.e. the site binding energies) whichcould also explain the differences in capacity.

The specific capacities vs. cycle number at different cycling rates(C/25, C/6, 1 C, 3 C, and 10 C) calculated from galvanostatic curves areshown in FIG. 31(C). The highest capacity was obtained at a rate ofC/25. The specific capacity values stabilize after 5 cycles, for allscan rates. At a C/25 rate, the capacity is 225 mAh·g⁻¹, whichcorresponds to y≈1. At rates of 1 C and 3 C, the capacities, after 80cycles, were, respectively, 110 mAh·g⁻¹ and 80 mAh·g⁻¹. Even at rates of10 C, a stable capacity of 70 mAh·g⁻¹ was obtained for more than 200cycles. These results clearly demonstrate that it is possible to stablyelectrochemically intercalate Li⁺ ions in the interlayer spaces betweenexfoliated Ti₂C sheets, and achieve stability.

The exfoliated Ti₂C, produced by HF treatment of Ti₂AlC powders, showedreversible capacity about 5 times higher than pristine Ti₂AlC, due toits open structure, weaker interlaminar forces, and higher SSA.Electrochemical measurements showed intercalation and deintercalation ofLi⁺ ions at 1.6 V and 2 V vs. Li⁺/Li, respectively. The exfoliated Ti₂Cmaterial exhibited a stable capacity of 225 mAh·g⁻¹ at a C/25 rate,corresponding to about one Li per Ti₂CO_(x) formula unit. A stablecycling capacity of 80 mAh·g⁻¹ was observed after 120 cycles at a 3 Crate, and 70 mAh·g⁻¹ was observed after 200 cycles at a 10 C rate.

Example 12 Intercalation of Lithium and Use of Other MXenes in Batteries

Similar experiments with Ti₃CN, TiNbC, Nb₂C, V₂C, and Ta₄C₃ have alsoshown that these materials can also be intercalated with Li and used inlithium ion batteries.

To explore the feasibility of using Nb₂CT_(s) and V₂CT_(s) as electrodesin lithium ion batteries (LIBs), cyclic voltammetry (CV) andgalvanostatic charge—discharge cycling (GV) were carried out. The CVcurves for Nb₂CT_(s) showed no significant lithiation and delithiationcapacity at voltages higher than 2.5 V. Hence, the GV for Nb₂CT_(s) wascarried out between 0 and 2.5 V against Li/Li⁺. The voltage profile forNb₂CT_(s) at 1 C cycling rate yielded a first cycle capacity of ˜422mA·h·g⁻¹. The second cycle capacity was about 250 mA·h·g⁻¹. Withoutintending to be bound by the correctness of any particular theory, thereason for the first cycle irreversibility could be due to solidelectrolyte interphase (SEI) formation or due to irreversible reactionof Li with the surface groups and/or water molecules in theas-synthesized MXene. In principle, this irreversibility could beminimized by controlling the surface chemistry of MXene or byprelithiating the electrode material. After 100 cycles, a reversiblecapacity of 170 mA·h·g⁻¹ was obtained.

Because the CV for V₂CT_(s) showed a large capacity close to 3 V, thismaterial was tested between 0 and 3 V against Li/Li⁺. The first cyclecapacity was found to be ˜380 mA·h·g⁻¹ and the reversible capacity ˜210mA·h·g⁻¹. Intriguingly, the V₂CT_(s), produced by etching attritionmilled V₂AlC, showed >30% enhancement in Li uptake compared to V₂CT_(s)produced from unmilled V₂AlC. This might be explained by the decreasedparticle size, facilitating Li diffusion between the layers. Areversible capacity of 288 mA·h·g⁻¹ was obtained instead of 210 mA·h·g⁻¹at the same cycling rate of 1 C after 50 cycles. A reversible capacityof 260 mA·h·g⁻¹ was obtained for the V₂CT_(s), produced by etchingattrition milled V₂AlC, after 150 cycles.

More than ⅔ of the reversible lithiation capacity for Nb₂CT_(s) wasbelow 1 V, while for both Ti₃C₂ and Ti₂C, the capacities below 1 V wereabout ½ of the reversible capacity. Conversely, in the case of V₂CT_(s),less than ½ of the reversible lithiation capacity is below 1 V and morethan ⅔ of the delithiation capacity is at voltages higher than 1.5 V.This is an important finding since it shows that each MXene has its ownactive voltage window. With the variety of possible MXenes chemistries,selection of an optimum MXene for a required voltage window can inprinciple be achieved. That is, some MXenes could function better asanodes, while others could, in principle, be used as cathode materialsfor lithium ion batteries. Both Nb₂CT_(s) and V₂CT_(s) (produced by HFtreatment of attrition milled V₂AlC powders at RT for 8 h) were shown tobe capable of handling high cycling rates. At 10° C., capacities of 110mA·h·g⁻¹ for Nb₂CT, and 125 mA·h·g⁻¹ for V₂CT_(s) were obtained after150 cycles. These values were much higher than what was reported forcommercial graphite when charged and discharged at 10° C. (graphiteloses more than 80% of its theoretical capacity at 10° C.). The highrate capability could be explained by the low Li diffusion barrier inMxenes. The coulombic efficiency at the reversible capacity was about99.6% for Nb₂CT_(s) at 10° C. For V₂CT_(s), the coulombic efficiencyvaried between 98% and 100%. Although the reversible capacity of MXenesat high cycling rates (i.e., 10° C.) was comparable to titania basedanodes, the latter have maximum theoretical capacities of the order of170 mA·h·g⁻¹ even at slow scan rates, while V₂CT_(s) (produced frommilled V₂AlC) has a reversible capacity of 260 mA·h·g⁻¹ at 1° C. Theresults obtained herein were obtained on just synthesized and not wellpurified compounds and should thus be considered quite preliminary. Thehigher rate performances, however, were encouraging and suggest thatNb₂CT_(s) and V₂CT_(s) can be used as promising electrode materials inlithium ion batteries, especially for high power applications. Forexample, the Li-capacities of additives-free fully delaminatedTi₃C₂T_(s) electrodes were roughly 4 times those of nondelaminatedTi₃C₂T_(s).

Example 13 Intercalation of Sodium, Potassium, Ammonium, Magnesium, andAluminum Ions

The examples provided herein for the intercalation of various ions useTi₃C₂T_(s) as a convenient template for investigation. It should beappreciated that the results described herein are expected to bereproducible with other MXene materials, and embodiments include thosewherein the intercalation is described more generally with respect tothese other MXene materials. That is, other specific embodiments includethe other MXene materials described herein intercalated with the ionsdescribed herein, and the articles derived from such intercalatedmaterials.

Example 13.1 Materials and Methods

Ti₃C₂T_(s) (where T_(s) stands for surface termination, such as OH, O orF bonded to Ti atoms) was synthesized by exfoliating the correspondingMAX phases with “A” element etched away. Ti₃AlC₂ powder with particlesize less than 38 μm was treated with 50% aqueous HF solution (FisherScientific, Fair Lawn, N.J.) at room temperature (RT), for 18 h. Theresulting suspensions were washed six to seven times using deionizedwater and separated from remaining HF by centrifuging until the pH ofthe liquid reached around 5. The wet sediment was moved to a wide-mouthjar by ethanol and dried in air for 3 to 4 days. Then the obtainedTi₃C₂T_(s) was placed into capped glass vials and stored at ambientconditions for further experiments.

Electrodes were prepared by mechanical processing of the pre-mixedslurry, containing ethanol (190 proof, Decon Laboratories, Inc.),Ti₃C₂T_(s) powder, polytetrafluoroethylene (PTFE) binder (60 wt. % inH₂O, Aldrich) and onion-like carbon (OLC) (28), which was added tocreate a conductive network in-between the particles (MXene isanisotropic: good in-sheet conductivity, poor conductivity between thesheets). Resulting electrodes which were used for all experimentscontained: 85 wt. % of the Ti₃C₂T_(s), 10 wt. % of OLC, 5 wt. % of PTFEand had thickness of 75-100 μm and mass density per unit area of 7-9mg/cm2. (1)

To intercalate Ti₃C₂T_(s), 0.15 g of the powder was suspended in 5 ml of30 wt. % aqueous solution of potassium hydroxide, potassium acetate,lithium acetate, sodium acetate, sodium formate, sodium citrate, andzinc sulfate; 25, 20 and 10 wt. % aqueous solution of magnesium sulfate,sodium sulfate and potassium sulfate, respectively; 30% aqueoussolutions of acetic acid, sulfuric acid, and ammonium hydroxide. Then,the mixtures were stirred for 24 h with a magnetic stirrer at roomtemperature, RT. Afterwards, the resulting colloidal solutions werefiltered through a polyester membrane (25 mm diameter, 3 μm pore size,Osmonics Inc., Minnetonka, Minn., USA) and dried in a desiccator undervacuum (<10 Torr) at RT.

To obtain few-layer Ti₃C₂T_(s), multilayered Ti₃C₂T_(s) was stirred withdimethyl sulfoxide (DMSO) for 18h at room temperature, then thecolloidal suspension was centrifuged to separate the intercalated powderfrom the liquid DMSO. After decantation of the supernatant, deionizedwater was added to the residue in a weight ratio of MXene to water of1:500. Then the suspension was sonicated under Ar for 4 h, andcentrifuged for 1 h with 3500 rpm. At last, the supernatant wasdecantated and filtered using a porous MF-millipore mixed celluloseester membrane filter (47 mm diameter, 0.025 μm pore size, FisherScientific, Fair Lawn, N.J., USA) and dried in a desiccator under vacuum(<10 Torr) at RT for 24 h, resulting in MXene paper that detaches easilyfrom the membrane 2 and can be further used as a free-standingelectrode. The thickness of the MXene paper varied from 2 to 20 μm. Massdensity per unit area of tested electrodes was 2-3 mg/cm2.

The following ionic compounds were used for intercalation intoTi₃C₂T_(s): potassium hydroxide (≧85.0%, Fisher Chemical, Fair Lawn,N.J., USA), potassium sulfate (certified ACS crystalline, FisherScientific, Fair Lawn, N.J., USA), potassium acetate (ACS reagent grade,MP Biomedicals, LLC, Solon, Ohio, USA), lithium acetate anhydrous (≧99%,Acros Organics, Fair Lawn, N.J., USA), sodium acetate anhydrous (≧99.0%,Alfa Aesar, Ward Hill, Mass., USA), sodium formate (>99.0%, Alfa Aesar,Ward Hill, Mass., USA), sodium citrate tribasic dehydrate (>98%, SigmaAldrich, St. Louis, Mo., USA), sodium sulfate anhydrous (99.7%, AcrosOrganics, Fair Lawn, N.J., USA), magnesium sulfate (≧99.5%, Alfa Aesar,Ward Hill, Mass., USA), zinc sulfate heptahydrate (≧99.0%, SigmaAldrich, St. Louis, Mo., USA). Ammonium hydroxide (28-30 wt. % in water,Fisher Scientific, Fair Lawn, N.J., USA), acetic acid (99.8%, AcrosOrganics, Fair Lawn, N.J., USA), and sulfuric acid (50%, Ricca ChemicalCompany, Arlington, Tex., USA) were also used as intercalants.

The following salts were used as electrolytes in electrochemicalexperiments: potassium hydroxide (≧85.0%, Fisher Chemical, Fair Lawn,N.J., USA), potassium sulfate (certified ACS crystalline, FisherScientific, Fair Lawn, N.J., USA), sodium acetate anhydrous (≧99.0%,Alfa Aesar, Ward Hill, Mass., USA), sodium hydroxide (≧98%, Alfa Aesar,Shore Road, Heysham, Lancs UK), sodium nitrate (≧99%, Sigma Aldrich, St.Louis, Mo., USA), magnesium nitrate hexahydrate (≧99%, Sigma Aldrich,St. Louis, Mo., USA), magnesium sulfate (≧99.5%, Alfa Aesar, Ward Hill,Mass., USA), aluminum sulfate hydrate (≧98.0%, Fluka, St. Louis, Mo.,USA), ammonium sulfate (≧99.0%, Sigma Aldrich, St. Louis, Mo., USA), andlithium sulfate (≧98.5%, Sigma Aldrich, St. Louis, Mo., USA).

Activated carbon film electrodes were prepared following the sameprocedure as for the Ti₃C₂T_(s) electrodes, but without any conductiveadditive. Resulting AC electrodes composition was 95 wt. % of YP-50activated carbon (Kuraray, Japan) and 5 wt. % of the PTFE. They hadthickness of 100-150 μm and mass density per unit area of 10-25 mg/cm².

All electrochemical measurements were performed in 3-electrode Swagelokcells, where MXene served as working electrode, over-capacitiveactivated carbon films were used as counter electrode and Ag/AgCl in 1 MKCl as a reference in order to precisely control electrochemicalpotentials.

Cyclic voltammetry, electrochemical impedance spectroscopy andgalvanostatic cycling were performed using a VMP3 potentiostat(Biologic, France).

Cyclic voltammetry was performed using scan rates from 1 mV/s to 1000mV/s. Diapasons of cycling were chosen using the following principles:

1) As starting potential, open circuit potential right after assembly ofthe cell was chosen.

2) Minimum potential was chosen by subsequent CV series with increasinglower limit, with the end at the lower limit minimum potential, at whichno electrolyte decomposition was observed.

The reason for choosing OCP as upper limit is to avoid oxidation of thematerial in aqueous electrolytes which would lead to higher resistanceand lower resulting capacitance (see FIG. 37).

Electrochemical impedance spectroscopy (EIS) was performed at opencircuit potential with a 10 mV amplitude between 10 mHz and 200 kHz.

Galvanostatic cycling was performed at 0.1 and 1 A/g with potentiallimits selected specifically for each electrolyte: from −0.5 to 1 V vs.Ag/AgCl for 1 M KCl, from 0 to −0.7 V vs. Ag/AgCl for 1 M MgSO₄ and 1 MNaOAc.

X-Ray diffraction patterns were recorded with a powder diffractometer(Rigaku SmartLab) using Cu Kα radiation (λ=1.54 Å) with 0.01° 2θ stepsand 6 s dwelling time. Scanning electron microscopy (SEM) and energydispersive X-ray spectroscopy (EDX) analysis were performed on ZeissSupra 50 VP (Carl Zeiss SMT AG, Oberkochen, Germany).

XRD patterns of the Ti₃C₂T_(s) electrodes were collected on a Brucker D8diffractometer using a Cu Kα radiation (λ=1.5406 Å) in the range2θ=5−20° with a step of 0.02°. The sample was placed in a 2-electrodeSwagelok-type cell and covered with a Mylar window to avoid electrolyteevaporation, allowing in-situ XRD recording (cell from LRCS, AmiensUniversity). A MXene film, a mixture of 90% Ti₃C₂T_(s), 5% PTFE and 5%carbon black served as the working electrode, and was pressed on anickel foam current collector and dried at 120° C. Over-capacitiveactivated carbon films were used as counter electrode. Cyclicvoltammetry advanced technique was used in order to control the cellpotential. The scans were recorded each 0.2 V after linear sweep at 1mV/s.

Theoretical specific surface area calculations for Ti₃C₂(OH)₂ andestimation of the number of layers in multilayer exfoliated Ti₃C₂T_(s)and few-layer Ti₃C₂T_(s):Area of one lattice=Lattice parameters a×b×sin)(60°)=3.0581 Å×3.0588Å×(3)^(0.5)/2×10⁻²⁰=8.1E−20 m².

Each layer in the cell has 3 Ti, 2 C, 2 O, and 2 H. Then the weight ofthe layer in the cell=[201.64 g/mole]/[6.023E²³ atoms/mole]=3.3478E−22g. The SSA=8.1×10⁻²⁰/3.3478×10⁻²²=241.97 m²/g (one side). Then the SSAof a Single layer (2 sides) of Ti₃C₂(OH)₂ will be 483.94 m²/g.

These calculations ignore the presence of edges and defects.

Experimental SSA for MXene paper and its corresponding number of layers:

98 m²/g using N₂ yielded ca. 5 layers

128 m²/g using CO₂ yielded ca. 4 layers

167 m²/g using Ar yielded ca. 3 layer

For stacked Ti₃C₂T_(s), experimental SSA calculated from nitrogensorption, is 23 m²/g, which translated to ca. 21 layers in an averageMXene lamella.

Calculations of volumetric power and energy densities of electrode andcell:C=(∫jdV)/s/V[F/cm³]E=0.5C*V ²/3600[Wh/cm³]P=E*s/V*3600[W/cm³]

where C-normalized capacitance [F/cm³], j-current density [A/cm³],s-scan rate [V/s], V-voltage window [V], similarly calculations of thegravimetric properties was performed, but gravimetric capacitance andcurrent density were used instead.

Example 13.2

A large number of salts, bases, and acids were explored under conditionsdescribed in the Schematic of FIG. 32. See also Table 2.

TABLE 2 Changes in c-lattice parameters acter intercalation ofTi₃C₂T_(s) with ions. Value of Δ (third column) indicates the increasein c-lattice parameter of Ti₃C₂T_(s) after intercalation (second column)compared to initial c value of 20.3 Å Intercalatant c, Å Δ,Intercalatants which possess a basic character when dissolved in waterPotassium hydroxide 25.4 5.1 Ammonium hydroxide 25.3 5.0 Sodiumcarbonate 25.3 4.8 Sodium hydroxide 25.1 4.6 Sodium formate 24.9 4.6Sodium citrate 24.9 4.5 Sodium acetate 24.8 4.3 Potassium acetate 24.64.2 Lithium acetate 24.5 Interalatants which possess a nearly neutralcharacter when dissolved in water Zinc sulfate 21.7 1.4 Potassiumsulfate 21.4 1.1 Magnesium sulfate 21.3 1.0 Sodium sulfate 21.0 0.7

X-ray diffraction (XRD) patterns showed that, after placing theTi₃C₂T_(s) in various salt solutions (FIG. 33, A to C), there was adownshift in the (0002) peak position. This downshift shows that in allcases, there was an increase in the c-lattice parameter. For example,the c value of Ti₃C₂T, increased from 20.3 Å to as much as 25.4 Å whenplaced in potassium hydroxide (KOH) and ammonium hydroxide (NH₄OH)solutions (FIG. 33A). In addition to the compounds listed in FIG. 33, Ato C, other salts intercalated spontaneously when the MXene powders wereimmersed in sodium carbonate (Na₂CO₃), sodium hydroxide (NaOH), orlithium hydroxide (LiOH) solutions.

Not all salts behaved similarly. In the case of high-pH solutions suchas KOH, NH₄OH, NaOH, LiOH, and several others (Table 2), the changes inthe interplanar spacing were large (FIG. 33A). Conversely,close-to-neutral solutions such as sodium, potassium, and magnesiumsulfates resulted in smaller changes in c (FIG. 33B; see also Table 2).No shift in the (0002) peak positions was observed when Ti₃C₂T_(s) wasimmersed in acetic or sulfuric acid.

To shed light on whether the cations or anions intercalated theTi₃C₂T_(s) layers, three sodium salts with differing anion radii weretested. The results (FIG. 33C) showed that the c-axis expansions werecomparable and independent of anion radii. Furthermore,energy-dispersive x-ray spectroscopy analysis of Ti₃C₂T_(s) aftertreatment in the different sulfate salts (FIG. 33B) confirmed thepresence of the cations; sulfur was not detected (Table 3), confirmingthat it is the cations that intercalate between the Ti₃C₂T_(s) layers.

TABLE 3 Energy-dispersive X-ray spectroscopy analysis of Ti₃C₂T_(x)powder before and after intercalation. Atomic % Cation of Material Ti CO F electrolyte S Ti₃C₂T_(x) 30.0 14.8  16.0  18.9  — — Ti₃C₂T_(x) + KOH30.0 21.2  30.9  11.4  3.2 — Ti₃C₂T_(x) + NaOAc 30.0 16.2  18.2  27.4 5.5 — Ti₃C₂T_(x) + K₂SO₄ 30.0 17.8  8.4 15.4  1.4 0.0 Ti₃C₂T_(x) +Na₂SO₄ 30.0 17.5  12.9  15.8  1.0 0.0 Ti₃C₂T_(x) + MgSO₄ 30.0 29.7 17.0  18.5  0.5 0.1 Ti₃C₂T_(x) + MgSO₄ 30.0 59.2* 33.5* 40.0* 2.0 0.0(electrode) *Values of the carbon, oxygen and fluorine content areapproximate, since spectra were collected from the rolled Ti₃C₂T_(x)electrode, which contained carbon additive (contributes to C and Ocontent) and PTFE binder (contributes to C, O and F content)

Materials with large specific surface area are typically needed toobtain large capacitances in carbon materials for EDLCs. However, at 23m²/g, the surface area of multilayer exfoliated Ti₃C₂T_(s) was low. Itfollows that if double-layer capacitance were the only operativemechanism, one would have expected the capacitance for this material tobe less than that of (for example) activated graphene by a factor of100. However, as noted above, intercalation capacitance can by farexceed double-layer capacitances calculated solely on the basis of amaterial's surface area. To test this idea, fabricated multilayerTi₃C₂T_(s) electrodes were made and tested in NaOH—, KOH— and LiOH—containing electrolytes using a standard three-electrode asymmetricalsetup with an Ag/AgCl reference electrode (FIG. 34). The resultingcyclic voltammograms (CVs) are shown in FIG. 35 [see FIG. 36 for thecorresponding electrochemical impedance spectroscopy (EIS) results]. Therectangular-shaped CVs indicate capacitive behavior in these basicsolutions. Note that in all experiments, the open circuit potential(OCP) was taken as the starting potential for the CV scans because 0.1 Vabove this potential, Ti₃C₂T_(s) oxidation is observed in aqueouselectrolytes (see FIG. 37).

To study the effect of a cation's valence on the electrochemicalperformance of multilayer exfoliated Ti₃C₂T_(s) electrodes, CV scanswere taken in 1M solutions of potassium and aluminum sulfates andnitrates (FIG. 35B and FIG. 36B). Clearly, the responses in the K⁺- andAl³⁺-containing solutions were distinctively different, confirming onceagain that the cations (and not the anions) are intercalating. The CVplots for K₂SO₄ were almost perfectly rectangular. Conversely, the CVdata for the more acidic (see Table 4) and less conductive Al₂(SO₄)₃electrolyte yielded capacitance values that were significantly lower,and the shape of the CV at 10 m V/s and the EIS results showed a higherresistance (FIG. 35B and FIG. 36B). To ensure that lower electrolyteconductivity did not limit the capacitive performance, Ti₃C₂T_(s) wastested in 1 M Al(NO₃)₃, which had a conductivity similar to that of 1 MK₂SO₄ (Table 4). Although the normalized capacitance did not increaseappreciably, the CV loops were definitely more rectangular (FIG. 35B),demonstrating the role of electrolyte conductivity.

TABLE 4 Electric conductivity of the aqueous electrolytes used inelectrochemical experiments Electrolyte Conductivity, mS/cm 1M NaOH 1411M KOH 191 0.5M LiOH 90 0.5M K₂SO₄ 100 1M (NH₄)₂SO₄ 114 1M Mg(NO₃)₃ 1151M MgSO₄ 51 1M Al₂(SO₄)₃ 30 1M Al(NO₃)₃ 110 3M NaOAc 79

Further evidence for cation intercalation and its beneficial effect oncapacitance comes from the observation that for some electrolytes, timewas needed to reach a steady state or maximum capacitance. For stronglybasic electrolytes (Table 3), such as KOH solutions, the rectangular CVplots were observed almost immediately and the capacitances did notchange with time or cycle number. For other electrolytes, however, therewas a slow and gradual increase in capacitance with time. For example,for salts such as MgSO₄, the CV area increased steadily with time andthe maximum capacity was reached only after 48 hours (see FIG. 38 andFIG. 39). Unlike what is observed for graphite, there was noirreversible capacitance loss during the first cycle for any of theelectrolytes studied.

The performance of the multilayer Ti₃C₂T_(s) in all tested electrolytesis summarized in FIG. 35C. The specific capacitances were calculated byintegrating the discharge portions of the CV plots. The results clearlyshowed responses that depended on the electrolytes used. Moreover, thecalculated capacitances were quite high for a material with such lowsurface area.

In situ XRD studies of the intercalation process during cycling showedthat electrochemical cycling led to insignificant changes in the cvalues. For example, when a Ti₃C₂T_(s) electrode was cycled in aKOH-containing electrolyte, the c values fluctuated within 0.33 Å as thepotential was scanned from −1 to −0.2 V (FIG. 40A). Interestingly, aslight shrinkage in c values was observed with increasing voltage.Similar behavior was observed when Ti₃C₂T_(s) was cycled inNaOAc-containing electrolyte (FIG. 41). Without being bound by thecorrectness of any particular theory, one explanation for thisobservation was that the positively charged ions incorporated inTi₃C₂T_(s) increased the electrostatic attraction between layers, in amanner analogous to what was observed for MnO₂ in other systems. WhenTi₃C₂T_(s) was electrochemically cycled in a MgSO₄-containing solution,the shift of the (0002) peak almost doubled relative to the KOH andNaOAc electrolytes (compare FIG. 40A and FIG. 40B). Here again, a slightshrinkage in c values was observed with increasing voltage.

To gain further insight into the capacitances and what influences them,MXene “paper” produced by filtering delaminated Ti₃C₂T_(s) was tested.This paper, with a specific surface area of 98 m²/g, was flexible,hydrophilic, additive-free, and conductive. When tested in KOH, the CVswere rectangular, similar to those obtained when multilayer Ti₃C₂T_(s)powder was used (compare FIG. 42A to FIG. 35A). Furthermore, the EISresults indicated that the Ti₃C₂T, paper-based capacitors were lessresistive (FIG. 42B) than those made with multilayer Ti₃C₂T_(s) (FIG.36A). This improved electrochemical response can be related to a numberof factors, such as the absence of a binder in the system, good contactbetween the restacked flakes in the paper, increased accessibility ofthe structure, and thinner electrodes.

As shown in FIG. 40C, the use of Ti₃C₂T_(s) paper electrodes instead ofmultilayer exfoliated Ti₃C₂T_(s) in some electrolytes (e.g., KOH andNaOAc) roughly doubled the gravimetric capacitance (see FIG. 43 for moreinformation about the performance of Ti₃C₂T_(s) paper in NaOAc andMgSO₄). Further, the volumetric capacitance values recorded forfew-layer Ti₃C₂T_(s) were on the order of 340 F/cm³ for KOH (FIG. 42Aand FIG. 44). Those values are much higher than those found foractivated graphene [60 to 100 F/cm³] or micrometer-thin carbide-derivedcarbon electrodes [180 F/cm³]. A capacitance retention test performed bygalvanostatic cycling at 1 A/g showed almost no degradation inperformance after 10,000 cycles (FIG. 42D).

Example 14 Intercalation of MXene Materials with KaoliniticIntercalators

The examples provided herein for the intercalation of various ions useTi₃C₂T_(s) as a convenient template for investigation. It should beappreciated that the results described herein are expected to bereproducible with other MXene materials, and separate embodimentsinclude those wherein the intercalation is also described with respectto these other MXene materials. That is, other specific embodimentsinclude the other MXene materials described herein intercalated with thematerials described herein, and the articles derived from suchintercalated materials.

Example 14.1 Methods and Materials

The following chemicals were used in this Example: titanium aluminumcarbide 211 (Ti₂AlC, >92 wt. % purity, 3-ONE-2, Voorhees, USA), titaniumcarbide (TiC, 99 wt. % purity, Johnson Matthey Electronic, N.Y., USA),hydrofluoric acid (HF, 48-51 wt. %, Acros Organics, Morris Plains, USA),hydrazine monohydrate (HM, N₂H₄·H₂O, >98.0 wt. % purity, TCI America,Portland, USA), N,N-dimethylformamide (DMF, ≧99 wt. %, Acros Organics,Morris Plains, USA), dimethylsulfoxide (DMSO, m.w. 78.13, MP BiomedicalInc., Solon, USA), urea (Fisher Scientific, Fair Lawn, USA), acetone(≧99+ wt. %, Acros Organics, Morris Plains, USA), ethyl alcohol (FisherScientific, Fair Lawn, USA) tetrahydrofuran, THF (≧99+ wt. %, AcrosOrganics, Morris Plains, USA), chloroform (99.8 wt. %, stabilized in0.5-1% ethanol, Sigma Aldrich, St. Louis, USA), toluene (f.w. 92.14,Fisher Chemical, Fair Lawn, USA), hexane (≧99 wt. %, Reagent Plus, SigmaAldrich, St. Louis, USA), thiophene (≧99+ wt. %, Sigma Aldrich, St.Louis, USA), formaldehyde (37% w/w, Fisher Chemical, Fair Lawn, USA).All chemicals were used as received without further purification.

Characterization.

X-ray diffraction (XRD) patterns were recorded with a powderdiffractometer (Siemens D500, Germany) using Cu K_(α) radiation with awavelength ˜1.54 Å, with 0.02° 2θ steps and 1 sec dwelling time. Ascanning electron microscope, (SEM, Zeiss Supra 50VP, Germany) was usedto obtain images of the particles. The 2-D sheets were also imaged witha transmission electron microscope, TEM, (JEOL JEM-2100, Japan) using anaccelerating voltage of 200 kV. The TEM samples were prepared bysuspending the powders in isopropanol, sonicating for 1 min and drying adrop of the mixture on a 200 mesh lacey-carbon-coated copper grid. AnXPS (PHI 5000, ULVAC-PHI, Inc., Japan) was used to analyze the surfaceelemental composition powders before and after intercalation. Theresistances of freestanding cold-pressed discs of non-intercalated andintercalated MXenes were measured using a four-probe technique (CascadeProbe Station CPS-1303-24 with 4-point probe head Alessi C4S-57, CascadeMicrotech, Inc., Beaverton, USA). A detailed experimental section(materials used and techniques of material preparation) can be found inthe Supplementary Information.

Synthesis of Ti₃AlC₂.

The Ti₃AlC₂ powder was synthesized from a mixture of Ti₂AlC with TiC ina 1:1 molar ratio. The mixture was ball milled for 24 h, heated at 10°C./min to 1350° C. in a tube furnace under Ar flow, and held in theseconditions for 2 h. After cooling, the lightly sintered brick wascrushed using a mortar and pestle.

Synthesis of MXene.

Non-sieved Ti₃AlC₂ powder was treated with HF solutions at roomtemperature (RT), for 22 h. The resulting suspensions were washed fivetimes using deionized water and centrifuged to separate the powder untilpH reached ˜4. The resulting black powder was divided into two portions.A small part of still wet material was used immediately forintercalation. The rest was dried under vacuum at 100° C. for 22 h,placed into capped glass vials and stored at ambient conditions forfurther experiments.

Intercalation of MXene.

To intercalate MXene, hydrazine monohydrate (HM) was used. Two types ofpowders were used: i) as-received, washed, wet MXene and, ii) type (i)powder dried at 100° C. for 22 h. The MXene powders were suspendedeither in HM or a 1:3 mixture of HM and DMF, and stirred for 24 h with amagnetic stirrer, either at RT or at 80° C.

In all cases, the weight ratio of HM:MXene was 10:1. When the treatmentinvolved only hydrazine monohydrate, the suspensions were filtered andwashed with ethanol. In the case of the mixture of HM and DMF, DMF wasused for washing instead of ethanol. The powders were then dried in adesiccator under vacuum, created by a water jet pump (the pressure inthe desiccator was <10 Torr), at RT for 24 h or in the vacuum (˜10⁻²Torr) oven at 120° C. for 24 h.

Other organic compounds were also tried for intercalation into MXene.Those included DMSO, urea, DMF, acetone, ethyl alcohol, THF, chloroform,toluene, thiophene, and formaldehyde. The procedure to synthesize theMXene intercalation compounds was the same in all cases: i) 9 weightedsamples of Ti₃C₂, 0.3 g each, were mixed with 5 ml of each organicsolvent (excluding urea), then stirred for 24 h at RT; ii) in case ofurea, 5 ml of 50 wt. % aqueous solution of urea was added to 0.3 g ofTi₃C₂ and stirred for 24 h at 60° C. Afterwards, the resulting colloidalsolutions were filtered and dried in a desiccator under vacuum at RT.

De-Intercalation of MXene.

To de-intercalate hydrazine/DMF, the reacted powder was carefullyweighed, placed in a graphite crucible and outgassed at RT in vacuum(˜10⁻⁶ Torr) for 24 h. While under vacuum, the powder was then heated to200° C. at 10° C./min, held at this temperature for 72 h and cooled toRT. The powder remained under vacuum for another 48 h before it wasretrieved. Between its removal from the furnace and re-weighing, thesample was exposed to ambient air for ˜3 min.

Preparation of Pressed MXene Discs.

The non-intercalated and intercalated MXene powders were cold pressed toa load corresponding to a stress of 0.8 GPa using manual hydraulicpellet press (Carver, Model 4350.L, Carver, Inc., Wabash, USA). Theresulting discs were 12.5 mm in diameter and in the range of 228-432 μmthick.

Example 14.2 Observations on the Results of Experiments IntercalatingKaolinitic Intercalators

Consistent with the apparent layered structure of MXene materials, theirintercalation behaviors appear to resemble that of clays. Numerouscompounds were studied for clay intercalation, such as formamide and itsderivatives, dimethyl sulfoxide (DMSO), urea, alkali metal salts,long-chain alkylamines, and others. Hydrazine monohydrate (HM) orN₂H₄.H₂O, is probably the most common intercalation agent for clays. Itintercalates into the interlayer space between the octahedral aluminumhydroxide and tetrahedral silica sheets, resulting in an expansion ofthe clay c-lattice parameter, c-LP, along [0001] from 7.2 to 10.3-10.4Å. As shown below, the intercalation of hydrazine and co-intercalationwith DMF between the Ti₃C₂ layers also resulted in an increase of the clattice parameter, in this case from 19.5 Å to 25.48 Å and 26.80 Å,respectively. Partial de-intercalation of hydrazine occurred by heatingthe intercalated Ti₃C₂ at 120° C. in vacuum and de-intercalation of bothhydrazine and DMF at 200° C. Comparison of molecular dynamics simulationand experimental results suggested that a nearly complete monolayer ofhydrazine is inserted between the Ti₃C₂ layers of the host.Intercalation of Ti₃C₂ with urea and dimethyl sulfoxide (DMSO), as wellas intercalation of Ti₃CN and TiNbC with hydrazine and DMSO demonstratedin this study, suggest a possibility of synthesis of dozens of newintercalation compounds based on carbides and carbonitrides oftransition metals.

The schematic of intercalation of HM into the MXene is shown in FIG.45A. The (001) X-ray diffraction (XRD) peaks of MXene most pronouncedbefore intercalation with HM and/or DMF, were still present afterintercalation, but shifted to lower 2 θ angles (FIG. 45B-C, FIG. 46).Table 5 summarizes the c-LPs values for the HM and HM in DMF treatedmaterials. The c-LP of the initial material was 19.5±0.1 Å, a value thatdoes not change much with post intercalation treatment. After theinitial powder was exposed to HM and HM in DMF at 80° C. for 24 h, thec-LPs increased to 25.48±0.02 Å and 26.8±0.1 Å, respectively. The largerc-LP increase in the latter case points to a synergistic effect when theHM was dissolved in DMF prior to its intercalation. Note that when DMFalone was used, the increase in c was very small (22.9±0.2 Å),suggesting limited intercalation.

When the HM intercalated powders were heated to 120° C., their c-LPsdecreased from 25.48 to 20.6±0.3 Å (FIG. 45B-iii and FIG. 45C-iii),signifying that the intercalation process was reversible. In contrast,heating the powders intercalated with HM and DMF to 120° C. resulted ina small reduction in c-LP (FIG. 45B-ii, FIG. 45C-ii). However, when thesame powder was vacuum dried at 200° C., the c-LP decreased to 20.1±0.5Å (FIG. 45B-ii, FIG. 45B-ii). It follows that the HM/DMF combination wasmore resistant to de-intercalation than HM alone. Without being bound bythe correctness of any particular theory, this could be due to a higherboiling point of DMF (153° C.) compared to that of HM (114° C.).

TABLE 5 c-lattice parameters, in Å, for non-intercalated MXene and MXenetreated with HM, HM and DMF, and dried in different conditions Non- MD -non- HM MD (N/C HM in DMF Intercalant intercalated intercalated (XRD)‡ratio 0.375) (XRD)‡ Initial material* 19.5 ± 0.1 19.85 ± 0.01 25.48 ±0.02 25.31 ± 0.04 26.8 ± 0.1 After drying @ 120° C. 19.5 ± 0.1 N/A 20.6± 0.3 N/A 26.0 ± 0.2 Vacuum drying @ 19.3 ± 0.2 N/A N/A N/A 20.1 ± 0.5200° C. *Prior to intercalation, the Ti₃C₂-based powder was dried at100° C. for 22 h. ‡Both HM and HM in DMF treatments were carried out at80° C. for 24 h.

The XRD consistently showed that MXene were intercalated with HM. Theabsence of XRD peaks corresponding to a c-LP of 19.5 Å (FIG. 45B-i, FIG.45C-i) implied that nearly entire space between MXene layers wasintercalated.

XPS spectra (FIG. 47) provided further evidence of intercalation. As itwas previously described, pure exfoliated Ti₃C₂ sample showed presenceof Ti—C and Ti—O bonds as well as OH groups suggested by the observedO1s peak around 530 eV. In case of intercalated samples, the N1s signalwas observed around 400 eV in XPS spectra of both Ti₃C₂ treated with HMand HM/DMF (insets in FIGS. 47A and B, respectively). As expected, nopeaks of nitrogen were detected in pure Ti₃C₂.

SEM images of the exfoliated Ti₃C₂ powders, before and after HMtreatment in DMF at 80° C. for 24 h, shown in FIG. 48A-B, respectively,confirmed that: (i) the MXene remains exfoliated after intercalation,and, (ii) the layers apparently thicken (FIG. 48B) by gluing monolayerstogether forming 20-50 nm thick lamellas. These structures were formedwhen organics act as adhesive ligaments, gluing MXene monolayerstogether.

The smaller fragments attached to the edges of a larger MXene particlesin FIG. 48B were debris that most probably were formed during the long(24 h) stirring with magnetic stirrer bar. Such debris has never beenobserved for non-intercalated Ti₃C₂. Their formation suggests adifferent mode of fracture of MXene layers after intercalation.

TEM images and corresponding SAED patterns of Ti₃C₂ intercalated with HMin DMF at 80° C. for 24 h (FIG. 48A-F) showed no significant changes inthe structure of the basal planes of the Ti₃C₂ after intercalation (FIG.48C and inset in FIG. 48D). When separate MXene sheets were observed,their SAED patterns (FIG. 48F) confirmed the same hexagonal crystalstructure of the basal planes of the intercalated sample typical forboth exfoliated Ti₃C₂MXene (inset in FIG. 48D) and non-exfoliatedTi₃AlC₂ MAX phase. Measurements of the d-spacings for the intercalatedmaterials gave the values 2.648 Å and 1.540 Å for the (0 110) and (1210) lattice planes, respectively. These values result in an a-LP of3.057 Å, a value that is in excellent agreement with the a-LP ofexfoliated Ti₃C₂ before intercalation, as well as non-exfoliatedTi₃AlC₂, viz. 3.058 Å.

Other potential intercalants were also tested. The following organiccompounds were tested: thiophene, ethanol, tetrahydrofuran,formaldehyde, chloroform, toluene, hexane, DMSO, and urea. Of these,only DMSO and urea resulted in an increase in the c-LPs from 19.5±0.1 Åto 35.04±0.02 Å and 25.00±0.02 Å, respectively (FIG. 49). Theintercalation of these compounds was in good agreement with datareported for kaolinite.

Interestingly, XRD patterns taken 3 weeks after the initial DMSOintercalation (FIG. 49-ii) showed an even larger downshift of the (002)peaks corresponding to a c-LP of 44.8±0.1 Å. Based upon thisobservation, together with the fact that MXene powders are highlyhygroscopic and over the same period of time they become increasinglywet, this further increase of c-LPs over time may be due theintercalation of H₂O from the ambient air into the pre-open interlayerspace of the intercalated MXene, followed by capillary condensation ofwater. This effect was only observed for the DMSO intercalated MXenepowders.

Although the results above were obtained on Ti₃C₂, other MXenes can beintercalated in a similar way. To demonstrate that intercalation was ageneral phenomenon rather than the exclusive property of the Ti₃C₂-basedMXene, the treatment of two other members of the MXene family, Ti₃CN andTiNbC, was carried out with HM. As in the case of Ti₃C₂, the shift ofthe major XRD peak to lower 2 θ angles (FIG. 50) confirmed theirintercalation. It is important to note that one of those phases was acarbonitride with the same general formula as Ti₃C₂ (M₃X₂), whereas theother one represented a different kind of MXene with the formula M₂X(TiNbC), consistent with the thinking that other MXenes can formintercalation compounds as well.

To further support the fact of intercalation, the resistivity ofnon-intercalated MXenes and MXenes treated with HM was measured (Table6). Expansion of the van der Waals gap between sheets requires energythat comes from charge transfer between the guest and MXene, and altersthe number of charge carriers, affecting the conductivity. As expected,the resistivity values of all intercalated samples were higher than thatof non-intercalated due to the increase of their c-LPs afterintercalation. The difference in magnitude of the resistivity increasefor different intercalated MXenes at relatively the same expansion mightbe partially explained by different number of MXene atomic layers. Incase of M₃X₂, the resistivity increased by an order of magnitude whereasthe increase by two orders is observed for M₂C compounds. It isimportant to note that the resistivity values might be affected bysample density and the pressure used to compress the discs. Forinstance, the sheet resistivity of non-intercalated Ti₃C₂ pressed at 0.8GPa reported in Table 6 was lower than that pressed at 1 GPa.

TABLE 6 Sheet resistivity, resistivity and density of cold-pressed discsfor different non-intercalated MXenes and MXenes treated with HydrazineMonohydrate (HM). Sheet Resistivity, Ω/□ Resistivity, Ωm Density, g/cm³Non- Non- Non- inter- HM inter- HM inter- HM Sample calated treatedcalated treated calated treated Ti₃C₂ 61 243 0.016 0.056 2.58 2.71 Ti₃CN43 711 0.011 0.249 3.22 2.90 Nb₂C 321 12806 0.139 4.977 3.75 3.47 TiNbC230 44661 0.092 17.471 3.67 3.01

The properties of intercalation compounds, including electricalconductivity discussed above, to a large extent were determined by theamount, arrangement and reactions of the guest molecules with the hostmaterial. In addition, the structure of the intercalant often provided akey to deciphering the intercalation mechanisms. However, as alluded toabove, this problem's complexity was illustrated by the large body ofliterature on the structure of HM intercalation in clays.

Concerning the co-intercalation of HM and DMF, only simple geometricalconsiderations were provided. The increase of c-LP by 7.3 Å over thenon-intercalated MXene, measured for this material after intercalationat room temperature (Table 5) could be explained by the insertion ofboth HM and DMF molecules. Based on the experimentally measured increasein MXene c-LPs, upon co-intercalation of HM and DMF, the insertion of 1N₂H₄ and 1 DMF molecule in a stack configuration into the interlayerspacing of MXene can be suggested. The resulting increase in c-LP (7.3Å) is, in this case, close to the sum of 2.4 Å and 5.0 Å—the changes inc-LPs reported for N₂H₄ and DMF intercalated kaolinites, respectively.As noted above, the reduction of this value to 6.5 Å after drying at120° C. (Table 5) could be attributed to the partial de-intercalation ofHM molecules, leaving behind DMF molecules.

Example 15 Sheet Resistivities and Contact Angles of MX-Ene Discs

To measure the sheet resistances and the contact angle of various MXenecompositions, MXene discs (25 mm in diameter, 300 μm thick) werecold-pressed from the reacted powders. The latter were placed in a dieand cold-pressed to a load corresponding to a stress of 1 GPa. Thesurface or sheet resistances of cold-pressed, free-standing MXene discswere measured using a four-probe technique (Cascade Probe StationCPS-1303-24 with 4-point probe head Alessi C4S-57, Cascade Microtech,Inc., Beaverton, USA).

Contact angle measurements of deionized water were also performed atroom temperature using the sessile drop technique. Ten microliter waterdrops were placed on the surfaces of cold-pressed MXene discs. Thecontact angles were measured from photographs taken with a CCD camerayielding an accuracy of approximately ±3°.

The densities of the cold-pressed discs of the various MXenecompositions (Table 7) varied between 2.91 g/cm₃ for Ti₂C to 6.82 g/cm³for Ta₄C₃. If one assumes the c lattice parameters listed in Table 1 andOH terminated surfaces of MXene sheets, then it is possible to calculatethe theoretical densities. The last row in Table 7 lists the measureddensities of the pressed discs. The numbers in parentheses list the % oftheoretical densities that range from 50 to ca. 65%.

The sheet resistivity and resistivities of the various MXene discs arealso shown in Table 7. The resistivity values are higher than the MAXphases before treatment (<10Ω/□) presumably because of the replacementof the A layers with OH and/or F. When it is assumed that surface groupsare similar in all of the exfoliated MAX phases, the difference in theresistivity between the different phases can be partially explained bythe different number of atomic layers (3, 5, and 7 for M₂X, M₃X₂, andM₄X₃ phases, respectively). It is important to note that the resistivityvalues reported in Table 7 should be significantly higher than singleMXene sheets because of the method by which the resistivity wasmeasured. For example, the resistivity of bulk sintered Ti₃AlC₂ is 0.39μΩm. When Ti₃AlC₂ powders were cold-pressed at 1 GP, their resistivityincreased to 1200 μΩ-m, a, roughly, 3000 time increase.

Contact angle measurement results for water droplets on the cold-presseddiscs of exfoliated phases are also listed in Table 7. These values arelower than those of the corresponding MAX phases—that were also measuredin this work on cold-pressed samples, which were around 60°. Thereduction in contact angle can be explained by the presence of OHsurface groups after the HF treatment. In contradistinction, graphenecan be transformed from superhydrophopic to superhydrophilic by alteringthe surface groups. The hydrophilicity of the MXenes would be anadvantage when using aqueous electrolytes in energy storage devices ordispersing in water and alcohols for further processing.

TABLE 7 Resistivity and Contact Angle of Water on Cold-PressedFree-Standing Discs for Different Exfoliated Phases and TheirDensitities Property Ti₂C TiNbC Ti₃CNx Ta₄C₃ Ti₃C₂ Resistivity, Ω/□ 339171 125 104 22 Resistivity, Ω m 0.068 0.052 0.037 0.021 0.005 Contactangle, deg 32 31 27 41 34 Density of cold 2.91 3.23 2.95 6.82 3.12pressed discs^(a), g/cm³ (62%) (52%) (64%) (53%) (60%) (% oftheoretical) ^(a)The densities were estimated from the dimensions andweights of the cold-pressed discs. Number in parentheses is relativetheoretical density assuming OH termination of the MX-ene surfaces andthe c parameters listed in Table 1.

Example 16 Transparent Conductive Films

The fabrication of ˜1×1 cm² Ti₃C₂T_(s) films by selective etching of Al,from sputtered epitaxial Ti₃AlC₂ films, in aqueous HF or NH₄HF₂ isdescribed herein. Films that were about 19 nm thick, etched with NH₄HF₂,transmit ˜90% of the light in the visible-to-infrared range and exhibitmetallic conductivity down to ˜100 K. Below 100 K, the films'resistivities increase with decreasing temperature and exhibit negativemagnetoresistances; both observations consistent with a weaklocalization phenomenon characteristic of 2D defective solid. Thisadvance opens the door for the use of MXenes in electronic, photonic andsensing applications.

The examples provided herein describe results derived from Ti₃C₂T_(s).It should be appreciated that the results described herein are expectedto be reproducible with other MXene materials, and separate embodimentsinclude those wherein properties are described with respect to theseother MXene materials. That is, other specific embodiments include theother MXene materials described herein resulting in properties analogousto those described herein, and the articles derived from suchintercalated materials.

The materials described here represent a departure from existingliterature in several ways: (1) they are produced as continuousepitaxial thin films; (2) In all previous studies, the etchant was HF.Here it is shown that ammonium bi-fluoride, NH₄HF₂ can be used instead;(3) the one-step synthesis of a MXene, intercalated with ammonia, isdemonstrated; (4) Availability of epitaxial films on transparent andinsulating sapphire substrates enabled the measurement of some of thefundamental physical properties, such as optical absorption, in a broadwavelength range, and the temperature dependence of conductivity andmagnetoresistance down to 2 K. These films show high transparency forwavelengths in the visible to infrared range.

Example 16.1 Methods and Materials

Synthesis of Ti₃C₂.

Two chemicals were used to etch, at room temperature, the Ti3AlC2 films.The first was 50% concentrated HF (Sigma Aldrich, Stockholm, Sweden).Samples of nominal thickness of 15, 28, 43, and 60 nm were etched for10, 15, 60 and 160 min, respectively. The second was 1 M NH₄HF₂ (SigmaAldrich, Stockholm, Sweden). Samples of the same thickness as thosementioned above were etched for 150, 160, 420, and 660 min,respectively. After etching, the samples were rinsed in deionized water,then in ethanol.

Optical and Electrical Characterization.

Transmittance values of the films were obtained using aspectrophotometer (Perkin Elmer Lambda 950 UV-Vis) with a 2-nm slitwidth and resolution. Spectra were corrected with both 100% and 0%transmittance background spectra. A bare sapphire substrate was used asa reference. The number of MXene layers obtained for FIG. 54B werecalculated by dividing the total film thicknesses by the c/2 where c isthe lattice parameters obtained from XRD.

Room-temperature resistivities were measured using a four-point probemethod. Three sheet-resistance measurements were taken for each sample.The errors reported in Table 8 were calculated. The resistivity wasobtained by multiplying the sheet resistance with the correspondingaverage film thickness.

The temperature-dependent in-plane resistivity measurements wereperformed in a Physical Property Measurement System (Quantum Design, SanDiego, USA) using an external current source (Keithley 6220, Ohio, USA)and nanovoltmeter (Keithley 2182A). A linear four-point probe geometrywas used. Gold wires were attached to the films using silver paint.Positive and negative currents were applied at each temperature toeliminate any thermal offsets. The MR measurements were performed withthe magnetic field—up to 10 T—applied out of the plane of the film.

Example 16.2 Results

The starting films used were 15 to 60 nm thick Ti₃AlC₂ films depositedonto sapphire (000 l) substrates by magnetron sputtering. FIG. 51A showsa schematic of the process starting from the sputter-deposition ofTi₃AlC₂. Prior to the deposition of the Ti₃AlC₂ layers, a TiC incubationlayer was formed on the sapphire. The latter is key to growing epitaxialTi₃AlC₂ films. This is followed by etching of the Al layers resulting in2D Ti₃C₂T_(s), layers where T_(s) stands for the surface —O, —OH, or —Fterminations resulting from the aqueous HF etchant (see Example 16.1).The Ti₃C₂ surfaces are presumed to be OH-terminated. A scanningtransmission electron microscopy (STEM) image of the interface betweenthe TiC incubation layer and Ti₃C₂T_(s) is shown FIG. 51B. The fact thatthe very first MXene layer has an ordered structure bodes well for theproduction of single layer MXene films.

In these experiments, ammonium bi-fluoride, NH₄HF₂ was used as anetchant reported to produce the MXene. As described above, otherbifluorides and in-situ HF precursors many also be used for thispurpose. The main advantages of the latter are reduced hazard, relativeto HF and milder etchant. A third advantage is the concomitantintercalation of cations during the etching process. For brevity's sake,hereafter these films will be referred to as Ti₃C₂T_(s)-IC, where the ICrepresents the intercalated species, viz. NH₃ and NH₄ ⁺ (see below).

A typical XRD pattern of an as-deposited Ti₃AlC₂ film (FIG. 52A, I)shows the (000l) peaks from Ti₃AlC₂, a TiC incubation layer and thesapphire substrate. The presence of only peaks corresponding tobasal-plane oriented Ti₃AlC₂ indicates epitaxial growth, a fact alsoconfirmed by transmission electron microscopy (TEM) and selected areaelectron diffraction (SAED) (FIG. 53A). The Ti₃C₂T_(s) XRD pattern (FIG.52A, II) on the other hand, shows a downshift in angle of the 000l peakscorresponding to an increase in the c lattice parameter from 18.6 Å forTi₃AlC₂ to 19.8 Å for Ti₃C₂T_(s). The latter value agrees with previouswork on Ti₃C₂T_(s) synthesized from Ti₃AlC₂ powders. The XRD pattern ofTi₃C₂T_(s)-IC (FIG. 52A, III), is similar to the other two, except thatnow c is further increased to 24.7 Å. Similar behavior was observed whenTi₃AlC₂ powders were intercalated with NH₄OH or NH₄F after HF etching.In both cases, the c lattice expansion was of the order of 25%. Theindependence of the increase in the c lattice parameter on the nature ofthe anion of the etching solution strongly suggests that the cations(NH₄ ⁺) and/or ammonia (NH₃), and not the anions, are the intercalatedspecies. Herein, the etching and intercalation occur in a single step.This is an important result because it considerably simplifies theintercalation process.

X-ray photoelectron spectroscopy (XPS) measurements were performed onthe various films in order to characterize their chemical states andatomic compositions. The XPS results, shown in FIG. 52B-D for films,with a nominal thickness of 60 nm, demonstrate a shift in the Ti 2p andC 1s (FIGS. 52B and C) toward higher binding energies for the titaniumcarbide species in Ti₃AlC₂, Ti₃C₂T_(s) and Ti₃C₂T_(s)-IC, compared tothose of binary TiC (shown in FIGS. 53B and C as vertical lines),indicating the change in the nature of bonding between the Ti and Catoms in Ti₃AlC₂ and the corresponding MXenes. The latter most likelyoccurs because valence electrons are withdrawn from the Ti atoms, andsubsequently from the C atoms, in the MXene layers by the surfacefunctional groups, as well as, from the interaction of the surface withthe intercalated compounds. The removal of Al is verified by thehigh-resolution spectra in the Al 2p region for Ti₃C₂T_(s) andTi₃C₂T_(s)-IC (FIG. 52D), in which a very weak Al signal most probablyoriginating from aluminum fluoride. The Ti₃AlC₂, Al 2p signalcorresponds to Al bonded to Ti, as well as, surface aluminum oxide.

The reactions of HF with Ti₃AlC₂ are postulated to include:Ti₃AlC₂+3HF═AlF₃+3/2H₂+Ti₃C₂  (1)Ti₃C₂+2H₂O═Ti₃C₂(OH)₂+H₂  (2)Ti₃C₂+2HF═Ti₃C₂F₂+H₂  (3)

Reaction (1) is followed by reactions (2) and (3), which result in OHand F terminated Ti₃C₂ surfaces or Ti₃C₂T_(s). The elemental ratioobtained from the analysis of high-resolution (XPS) spectra isTi₃C_(2.2)O₂F_(0.6). As indicated by XPS, terminal hydroxyl and fluoridegroups exist on the surface of the material, thereby indirectlyconfirming the aforementioned reactions. EDX mapping in the TEM alsoconfirms the presence of F and O atoms between the Ti₃C₂ layers.

As discussed above for NH₄HF₂ etched Ti₃AlC₂, intercalation of ammoniumspecies between the resulting Ti₃C₂T_(s) layers occurs concomitantly tothe etching of the Al layers. It is thus reasonable to conclude that inthis case the following reactions are operative:Ti₃AlC₂+3NH₄HF₂=(NH₄)₃AlF₆+Ti₃C₂+3/2H₂  (4)Ti₃C₂ +aNH₄HF₂ +bH₂O═(NH₃)_(c)(NH₄)_(d)Ti₃C₂(OH)_(x)F_(y)  (5)

Unlike HF etching, etching with NH₄HF₂ results in formation of(NH₄)₃AlF₆ according to reaction (4). Reaction (5) depicts theintercalation of NH₃ and NH₄ ⁺ between the Ti₃C₂T_(s) layers 30. Inorder to confirm the nature of the intercalating species inTi₃C₂T_(s)-IC, a high-resolution XPS spectrum of the N 1s region wasrecorded (FIG. 52E). The latter was best fitted by two components: onefor NH₄ ⁺ (55.8% of N 1s; peak position: 402 eV, FWHM: 1.8 eV) 30; theother for NH₃ (44.2% of N 1s; peak position: 400.1 eV, FWHM: 1.8 eV) 28,29. It is, thus, reasonable to conclude that both species intercalatedthis MXene.

The elemental ratio obtained from the analysis of high-resolution XPSspectra of Ti₃C₂ produced by NH₄HF₂ etching of Ti₃AlC₂ wasTi₃C_(2.3)O_(1.2)F_(0.7)N_(0.2). Here again, the XPS analysis indicatedthe presence of terminal hydroxyl and fluoride groups.

Cross-sectional scanning TEM micrographs of as-deposited Ti₃AlC₂ films,before (FIGS. 53A and D) and after etching with HF (FIGS. 53B and E) orNH₄HF₂ (FIGS. 53C and F) clearly showed the presence of the TiCincubation layers and the effects of etching on the films'microstructures. The SAED patterns confirmed the out-of-plane epitaxialrelationship Ti₃AlC₂(0001)//TiC(111)//Al₂O₃(0001). At 18.6 Å, the clattice parameter for Ti₃AlC₂, obtained from the SAED pattern and TEMmicrographs, was in excellent agreement with that calculated from XRD(18.6 Å). At 19.5-20 Å, the c lattice parameter of Ti₃C₂T_(s) obtainedfrom SAED pattern also matches the one from XRD (19.8 Å). At 21 Å, the cfor Ti₃C₂T_(s)-IC measured from the SAED pattern is considerably lowerthan that obtained from XRD (25 Å). The most probable reason for thisstate of affairs is the de-intercalation of the ammonium species duringTEM sample preparation and/or observation.

The light elements of the surface termination groups (O, H and F) cannotbe seen between the layers, but the larger and non-uniform spacings seenin FIGS. 53B, C, E and F indirectly confirmed the weak interactionsbetween the MXene layers after etching and the formation of a 2Dstructure. The non-uniform interlayer spacing observed in the STEMimages of the HF-etched sample (FIG. 53B) could also account for thepeak broadening observed in XRD (FIG. 52A).

Prior to etching, the initial thicknesses of the films examined in theTEM were 60 nm (FIG. 53A). However, as a result of the increase in c andthe separation between the MXene layers, due to exfoliation, the etchedfilms were thicker than the initial films (Table 8). Comparing theatomic layers in Ti₃C₂T_(s)-IC (FIGS. 53C and F) to those of theTi₃C₂T_(s) layers (FIGS. 53B and E), it was obvious that the former weremore uniformly spaced. This result most probably reflected the mildernature of NH₄HF₂ as compared to HF. For the latter the reaction isfaster (Table 8) and more vigorous than the former. Another possibleexplanation was that the intercalation of ammonia species led tostronger interactions between MXene layers, essentially “gluing” themtogether as observed for other MXene intercalation compounds.

In terms of light transmittance, both Ti₃C₂T_(s) and Ti₃C₂T_(s)-IC filmswere significantly more transparent than Ti₃AlC₂ of the same initialthickness, 15 nm, (Table 8 and FIG. 54A). The increased transparency ofTi₃C₂T_(s) and Ti₃C₂T_(s)-IC, compared to that of Ti₃AlC₂ was alsoevident visually.

With 90% transmittance, the Ti₃C₂T_(s)-IC films were the mosttransparent, followed by the Ti₃C₂T_(s) films at 70%. With atransmittance of 30%, the Ti₃AlC₂ films were the least transparent. Itis worth noting here that the transmittance of all films would have beenhigher had the TiC incubation layer been absent.

A linear dependence of the absorbance—that is independent of thewavelength of the light—on the thickness of the Ti₃C₂T_(s) andTi₃C₂T_(s) IC films was observed (FIG. 54B). The similarities in thetransmittance curves and the linear dependencies of absorbance valuesfor both samples, suggest similar structures for Ti₃C₂T_(s) andTi₃C₂T_(s)-IC. A crude estimation of the transmittance of a single MXenelayer, d, (since each length c is comprised of two MXene layers, d isapproximately equal to the film thickness divided by 2c) could beobtained from the linear fits of absorbance vs. d. The transmittances,calculated thusly, at a wavelength of 240 nm, for single layers ofTi₃C₂T_(s) and Ti₃C₂T_(s)-IC are about 90.5% and 91.5%, respectively;the corresponding transmittances, at a wavelength of 800 nm, are 97.3%and 97.1% respectively. The latter values are quite close to thosereported for graphene single layers. Note that to obtain theaforementioned values, both the thickness and absorbance of the TiC seedlayer were neglected.

TABLE 8 Thickness, etching duration, resistivity, and lighttransmittance - at a wavelength of 700 nm - of unetched and etchedTi₃AlC₂ thin films. Each set had the same nominal thickness beforeetching Deposition Etching Time Thickness, duration, Resistivity,Transmittance, (minutes) nm min μΩ.m % Set 1 Ti₃AlC₂ 5 15.2 ± 0.5^(a)0.45 ± 0.41 31 Ti₃C₂T_(s) 17.2 ± 0.8^(a) 9.5 39.23 ± 1.21  68Ti₃C₂T_(s)-IC 18.7 ± 0.6^(a) 150 4472.75 ± 323    85 Set 2 Ti₃AlC₂ 1027.7 ± 0.8^(a) 0.34 ± 0.01 14 Ti₃C₂T_(s) 28.4 ± 1.8^(a) 15 2.28 ± 0.0449 Ti₃C₂T_(s)-IC 31.3 ± 1.2^(a) 160 5.01 ± 0.03 37 Set 3 Ti₃AlC₂ 20 43.4± 3.6^(b) 0.31 ± 0.01 5.2 Ti₃C₂T_(s) 47.1 ± 3.5^(b) 60 22.27 ± 0.43  30Ti₃C₂T_(s)-IC 52.8 ± 2.5^(b) 420 30.90 ± 2.79  28 Set 4 Ti₃AlC₂ 30 60.0± 5.4^(c) 0.35 ± 0.01 3.4 Ti₃C₂T_(s) 67.4 ± 0.5^(c) 160 1.76 ± 0.02 15Ti₃C₂T_(s)-IC 74.7 ± 0.5^(c) 660 54.01 ± 4.51  14 ^(a)Determined by XRR.^(b)Interpolated. ^(c)Obtained from direct measurement in TEM (FIG.53A-B). ^(d) Obtained from direct measurement in TEM after accountingfor the decrease in thickness due to partial de-intercalation (FIG.53C).

The electrical properties also confirm the metallic-like nature of theconductivities of all films despite their optical transparency. TheTi₃AlC₂ films were metallic with resistivity, ρ, values in the range of0.37 to 0.45 μΩm. The latter increased linearly with increasingtemperature (FIG. 55A). Furthermore ρ increases with decreasing filmthickness (Table 8). The resistivity values of the Ti₃C₂T_(s)-IC filmsare systematically higher than those produced by HF etching. Forinstance, 28 nm nominally thick Ti₃C₂T_(s) and Ti₃C₂T_(s)-IC films haveρ values of 2.3 and 5.0 μΩm, respectively. This result is alsoconsistent with the resistivities of MXenes intercalated with organiccompounds. The ρ's of the etched films also depend significantly onetching time; longer etching times lead to higher ρ values. Note thatthe results listed in Table 8 were those obtained upon the full MAX toMXene conversion. The latter was determined by intermittent etching ofeach film, followed by XRD. When the Ti₃AlC₂ peaks disappeared, theetching process was arrested.

At 1.8 μΩm, a 60 nm nominally thick Ti₃C₂T_(s) sample was the mostconductive of the HF etched films Ti₃C₂T_(s) films (Table 8). However,at 15%, its transmittance was poor. The 15 nm nominally thick Ti₃C₂T_(s)sample exhibited the highest transmittance (68%) with a ρ of 39.2 μΩm.For the Ti₃C₂T_(s)-IC films, the lowest resistivity was 5.0 μΩm, with atransmittance of about 37%. The most transparent (>85%) Ti₃C₂T_(s) filmhad a resistivity of ≈4.5 mΩm. And while this transmittance value wascomparable to ITO, the sheet resistance was roughly an order ofmagnitude higher than ITO films having the same transmittance. Thehigher resistivities observed here may be due to the morphology of theas-deposited films. While Ti₃AlC₂ films were predominantly c-axisoriented (FIG. 51A), there is also a secondary grain population whereinthe basal planes are not parallel to the substrate. If the conductivityalong [0001] was significantly lower than that along the basal planes,such grains, when etched, will act as insulating islands. Reducing thefraction of such grains should result in films that are more conductivewhen etched.

It is predicted theoretically that altering the terminal bonds wouldalter the electronic properties of MXenes. Pure Ti₃C₂ was predicted tohave a metallic behavior, whereas Ti₃C₂F₂ and Ti₃C₂(OH)₂ were predictedto have band gaps of 0.05 and 0.1, respectively. Thus, there is room forenhancement of the conductivity of Ti₃C₂ by eliminating the surfacegroups and/or enhancing the quality of the films.

To elucidate the conduction mechanisms of the MXene layers, theirresistivities and magnetoresistances (MRs) from room temperature down toabout 2.5 K were measured. FIG. 55A shows the temperature dependentresistivity for Ti₃AlC₂, Ti₃C₂T_(s) and Ti₃C₂T_(s)-IC films of 28 nmnominal thickness. The Ti₃AlC₂ film exhibited metallic behavior from 300K down to about 10 K. For the Ti₃C₂T_(s) and Ti₃C₂T_(s)-IC films, on theother hand, metallic behavior was observed from 300 to about 100 K;below 100 K the resistivity increases with decreasing temperature (FIG.55B). Similar low-temperature behavior was observed in other Ti₃C₂T_(s)and Ti₃C₂T_(s)-IC films. The low temperature transport data can best befit assuming ρ˜ln T (inset in FIG. 55B).

As those skilled in the art will appreciate, numerous modifications andvariations of the present invention are possible in light of theseteachings, and all such are contemplated hereby. For example, inaddition to the embodiments described herein, the present inventioncontemplates and claims those inventions resulting from the combinationof features of the invention cited herein and those of the cited priorart references which complement the features of the present invention.Similarly, it will be appreciated that any described material, feature,or article may be used in combination with any other material, feature,or article, and such combinations are considered within the scope ofthis invention.

The disclosures of each patent, patent application, and publicationcited or described in this document are hereby incorporated herein byreference, in their entirety.

What is claimed:
 1. A composition comprising at least one layer havingfirst and second surfaces, each layer comprising: a substantiallytwo-dimensional array of crystal cells, each crystal cell having anempirical formula of M_(n+1)X_(n), such that each X is positioned withinan octahedral array of M, wherein M is at least one Group IIIB, IVB, VB,or VIB metal, wherein each X is C, N, or a combination thereof; n=1, 2or 3; and wherein at least one of said surfaces of each layer hassurface terminations comprising alkoxide, carboxylate, halide,hydroxide, hydride, oxide, sub-oxide, nitride, sub-nitride, sulfide,thiol, or a combination thereof.
 2. The composition of claim 1comprising a plurality of layers, wherein both surfaces of each layerhave said surface terminations.
 3. The composition of claim 1, wherein Mis at least one Group IVB, Group VB, or Group VIB metal.
 4. Thecomposition of claim 1 wherein M is Ti, and n is 1 or
 2. 5. Thecomposition of claim 1 wherein M_(n+1)X_(n) comprises Sc₂C, Sc₂N, Ti₂C,Ti₂N, V₂C, V₂N, Cr₂C, Cr₂N, Zr₂C, Zr₂N, Nb₂C, Nb₂N, Hf₂C, Hf₂N, Ti₃C₂,Ti₃N₂, V₃C₂, Ta₃C₂, Ta₃N₂, Ti₄C₃, Ti₄N₃, V₄C₃, V₄N₃, Ta₄C₃, Ta₄N₃, or acombination thereof.
 6. The composition of claim 1 wherein M_(n+1)X_(n)comprises Ti₃C₂, Ti₃CN, Ti₂C, Ta₄C₃ or (V_(1/2)Cr_(1/2))₃C₂.
 7. Thecomposition of claim 1 wherein M is Ta, and n is 2 or
 3. 8. Thecomposition of claim 1, the crystal cells having an empirical formulaTi₃C₂ or Ti₂C and wherein at least one of said surfaces of each layerhas surface terminations comprising hydroxide, oxide, sub-oxide, or acombination thereof.
 9. The composition of claim 1, wherein thecomposition comprises an electrically conductive or semiconductivesurface.
 10. The composition of claim 1, wherein M is at least one ofSc, Y, Lu, Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, or W.
 11. A stacked assemblyof at least two layers, each layer having first and second surfaces,each layer comprising: a substantially two-dimensional array of crystalcells, each crystal cell having the empirical formula of M_(n+1)X_(n),such that each X is positioned within an octahedral array of M; whereinM is a Group IIIB, IVB, VB, or VIB metal; each X is C, N, or acombination thereof; and n=1, 2, or 3; wherein the layers arecharacterized as having an average surface area and an averageinterlayer distance; and wherein at least one of said surfaces of eachlayer has bound thereto surface terminations comprising alkoxide,carboxylate, halide, hydroxide, hydride, oxide, sub-oxide, nitride,sub-nitride, sulfide, thiol, or a combination thereof.
 12. The stackedassembly of claim 11, wherein M_(n+1)X_(n) comprises Sc₂C, Sc₂N, Ti₂C,Ti₂N, V₂C, V₂N, Cr₂C, Cr₂N, Zr₂C, Zr₂N, Nb₂C, Nb₂N, Hf₂C, Hf₂N, Ti₃C₂,Ti₃N₂, V₃C₂, Ta₃C₂, Ta₃N₂, Ti₄C₃, Ti₄N₃, V₄C₃, V₄N₃, Ta₄C₃, Ta₄N₃, or acombination thereof.
 13. The stacked assembly of claim 12, whereinM_(n+1)X_(n) comprises Ti₂C, Ti₂N, Ti₃C₂, Ti₃N₂, Ti₄C₃, Ti₄N₃, Ta₃C₂,Ta₃N₂, Ta₄C₃, or Ta₄N₃, or a combination thereof.
 14. The stackedassembly of claim 13, wherein M_(n+1)X_(n) comprises Ti₃C₂, TiNbC, Nb₂C,Ti₃CN, Ti₂C, Ta₄C₃, or (V_(1/2)Cr_(1/2))₃C₂.
 15. The stacked assembly ofclaim 11, wherein M_(n+1)X_(n) is Ti₃C₂, TiNbC, Ti₃CN, or Ti₂C.
 16. Thestacked assembly of claim 15, wherein M_(n+1)X_(n) is Ti₃C₂.
 17. Thestacked assembly of claim 11, wherein the number of layers is in therange of 2 to about
 50. 18. The stacked assembly of claim 11, whereinthe average surface area of the layers is in the range of from about 100nm² to about 10,000 nm² or from about 100 μm² to about 10,000 μm². 19.The stacked assembly of claim 11, wherein lithium atoms, lithium ions,or both lithium atoms and lithium ions are intercalated between at leastsome of the layers.
 20. An energy-storing device or electrode comprisingthe stacked assembly of claim
 11. 21. A method of preparing acomposition comprising: removing substantially all of the A atoms from aMAX-phase composition having an empirical formula of M_(n+1)AX_(n);wherein M is at least one Group IIIB, IVB, VB, or VIB metal; wherein Ais an A-group element; each X is C, N, or a combination thereof; andn=1, 2, or 3, thereby providing a composition comprising at least onelayer having a first and second surface, each layer comprising asubstantially two-dimensional array of crystal cells; each crystal cellhaving an empirical formula of M_(n+1)X_(n), such that each X ispositioned within an octahedral array of M; and wherein at least one ofsaid surfaces of each layer has surface terminations comprisingalkoxide, carboxylate, halide, hydroxide, hydride, oxide, sub-oxide,nitride, sub-nitride, sulfide, thiol, or a combination thereof.
 22. Themethod of claim 21, wherein the A atoms are removed by a processcomprising a treatment with a fluorine-containing acid.
 23. The methodof claim 22, wherein the fluorine-containing acid is aqueoushydrofluoric acid.
 24. The method of claim 22, further comprisingsonication.
 25. The method of claim 24, wherein sonication is done usingultrasonic or megasonic sources.
 26. The method of claim 21, whereinremoving substantially all of the A atoms from a MAX-phase compositionis done electrochemically.